Allow for emission of swift.extension symbols for extensions to external types in swiftSymbolGraphGen

This includes:
 - introduction of the "swift.extension" symbol and "extensionTo" relationship
 - adding support for ExtensionDecl to the Symbol class
 - adding a "typeKind" field to the symbol's extension mixin which indicates what kind
   of symbol was extended
 - intoduction of the -emit-extension-block-symbols flag, which enables the behavior
   outlined below
 - adaptions to SymbolGraphASTWalker that ensure a swift.extension symbol is emitted
   for each extension to a type that does not exist in the local symbol graph
 - adaptions to SymbolGraph and SymbolGraphASTWalker that ensure relationships are
   correctly associated with the swift.extension symbol instead of the original type
   declaration's (extended nominal's) symbol where applicable
 - adaptions to SymbolGraphASTWalker that ensure swift.extension symbols are connected
   to their respective extended nominal's symbol using an extensionTo relationship

Testing:
- adds SymbolGraph tests that test behavior only relevant in
  -emit-extension-block-symbols mode
- adapts some SymbolGraph tests to additionally test similar behavior for
  extensions to external types in -emit-extension-block-symbols mode
- adapts some SymbolGraph tests to (additionally or exclusively) test the
  behavior with -emit-extension-block-symbols mode enabled

Bugfixes:
- fixes a bug where some conformsTo relationships implicated by the conformances
  declared on an extension to an external type were not emitted
  (see test/SymbolGraph/Relationships/ConformsTo/Indirect.swift)

Further changes:
- documents the strategy for naming and associating children declared in extensions
  to typealiases (see test/SymbolGraph/Relationships/MemberOf/Typealias.swift,
  test/SymbolGraph/Symbols/Names.swift)

Author: theMomax

include/swift/Option/Options.td

@@ -1330,6 +1330,11 @@ def pretty_print: Flag<["-"], "pretty-print">,
   Flags<[SwiftAPIExtractOption, SwiftSymbolGraphExtractOption]>,
   HelpText<"Pretty-print the output JSON">;
 
+def emit_extension_block_symbols: Flag<["-"], "emit-extension-block-symbols">,
+  Flags<[SwiftSymbolGraphExtractOption, FrontendOption,
+        NoInteractiveOption, SupplementaryOutput, HelpHidden]>,
+  HelpText<"Emit 'swift.extension' symbols for extensions to external types instead of directly associating members and conformances with the extended nominal when generating symbol graphs">;
+
 // swift-symbolgraph-extract-only options
 def output_dir : Separate<["-"], "output-dir">,
   Flags<[NoDriverOption, SwiftSymbolGraphExtractOption, SwiftAPIDigesterOption,

include/swift/SymbolGraphGen/SymbolGraphOptions.h

@@ -48,6 +48,11 @@ struct SymbolGraphOptions {
 
   /// Whether to include documentation for clang nodes or not.
   bool IncludeClangDocs = false;
+
+  /// Whether to emit "swift.extension" symbols for extensions to external types
+  /// along with "extensionTo" relationships instead of directly associating
+  /// members and conformances with the extended nominal.
+  bool EmitExtensionBlockSymbols = false;
 };
 
 } // end namespace symbolgraphgen

lib/Driver/ToolChains.cpp

@@ -616,6 +616,7 @@ ToolChain::constructInvocation(const CompileJobAction &job,
     context.Args.AddLastArg(Arguments, options::OPT_emit_symbol_graph_dir);
   }
   context.Args.AddLastArg(Arguments, options::OPT_include_spi_symbols);
+  context.Args.AddLastArg(Arguments, options::OPT_emit_extension_block_symbols);
   context.Args.AddLastArg(Arguments, options::OPT_symbol_graph_minimum_access_level);
 
   return II;
@@ -1110,6 +1111,7 @@ ToolChain::constructInvocation(const MergeModuleJobAction &job,
   context.Args.AddLastArg(Arguments, options::OPT_emit_symbol_graph);
   context.Args.AddLastArg(Arguments, options::OPT_emit_symbol_graph_dir);
   context.Args.AddLastArg(Arguments, options::OPT_include_spi_symbols);
+  context.Args.AddLastArg(Arguments, options::OPT_emit_extension_block_symbols);
   context.Args.AddLastArg(Arguments, options::OPT_symbol_graph_minimum_access_level);
 
   context.Args.AddLastArg(Arguments, options::OPT_import_objc_header);

lib/DriverTool/swift_symbolgraph_extract_main.cpp

@@ -171,6 +171,7 @@ int swift_symbolgraph_extract_main(ArrayRef<const char *> Args,
       ParsedArgs.hasArg(OPT_skip_inherited_docs),
       ParsedArgs.hasArg(OPT_include_spi_symbols),
       /*IncludeClangDocs=*/false,
+      ParsedArgs.hasArg(OPT_emit_extension_block_symbols),
   };
 
   if (auto *A = ParsedArgs.getLastArg(OPT_minimum_access_level)) {

lib/Frontend/CompilerInvocation.cpp

@@ -1240,6 +1240,8 @@ static void ParseSymbolGraphArgs(symbolgraphgen::SymbolGraphOptions &Opts,
 
   Opts.SkipInheritedDocs = Args.hasArg(OPT_skip_inherited_docs);
   Opts.IncludeSPISymbols = Args.hasArg(OPT_include_spi_symbols);
+  Opts.EmitExtensionBlockSymbols =
+      Args.hasArg(OPT_emit_extension_block_symbols);
 
   if (auto *A = Args.getLastArg(OPT_symbol_graph_minimum_access_level)) {
     Opts.MinimumAccessLevel =

lib/SymbolGraphGen/Edge.h

@@ -104,7 +104,20 @@ struct RelationshipKind {
   static inline RelationshipKind OptionalRequirementOf() {
     return RelationshipKind { "optionalRequirementOf" };
   }
-  
+
+  /**
+   A symbol A extends a symbol B with members or conformances.
+
+   This relationship describes the connection between extension blocks
+   (swift.extension symbols) and the type they extend.
+
+   The implied inverse of this relationship is a symbol B that is extended
+   by an extension block symbol A.
+   */
+  static inline RelationshipKind ExtensionTo() {
+    return RelationshipKind{"extensionTo"};
+  }
+
   bool operator==(const RelationshipKind &Other) const {
     return Name == Other.Name;
   }

lib/SymbolGraphGen/JSON.cpp

@@ -72,6 +72,27 @@ void swift::symbolgraphgen::serialize(const ExtensionDecl *Extension,
       if (const auto *ExtendedModule = ExtendedNominal->getModuleContext()) {
         OS.attribute("extendedModule", ExtendedModule->getNameStr());
       }
+
+      StringRef Kind;
+      switch (ExtendedNominal->getKind()) {
+      case DeclKind::Class:
+        Kind = "class";
+        break;
+      case DeclKind::Struct:
+        Kind = "struct";
+        break;
+      case DeclKind::Enum:
+        Kind = "enum";
+        break;
+      case DeclKind::Protocol:
+        Kind = "protocol";
+        break;
+      default:
+        Kind = "unknown";
+        break;
+      }
+
+      OS.attribute("typeKind", Kind);
     }
 
     SmallVector<Requirement, 4> FilteredRequirements;