[Explicit Module Builds] Add 'ClangImporter' setting for explicitly-built modules

This will mean that '-disable-implicit-swift-modules' also automatically implies two things:
1. Clang modules must also be explicit, and the importer's clang instance will get '-fno-implicit-modules' and '-fno-implicit-module-maps'
2. The importer's clang instance will no longer get a '-fmodules-cache-path=', since it is not needed in explicit builds

Author: artemcm

include/swift/Basic/LangOptions.h

@@ -878,6 +878,10 @@ namespace swift {
     /// and completely bypass the Clang driver.
     bool DirectClangCC1ModuleBuild = false;
 
+    /// Disable implicitly-built Clang modules because they are explicitly
+    /// built and provided to the compiler invocation.
+    bool DisableImplicitClangModules = false;
+
     /// Return a hash code of any components from these options that should
     /// contribute to a Swift Bridging PCH hash.
     llvm::hash_code getPCHHashComponents() const {

include/swift/Frontend/FrontendOptions.h

@@ -322,8 +322,8 @@ class FrontendOptions {
   /// By default, we include ImplicitObjCHeaderPath directly.
   llvm::Optional<std::string> BridgingHeaderDirForPrint;
 
-  /// Disable implicitly built Swift modules because they are explicitly
-  /// built and given to the compiler invocation.
+  /// Disable implicitly-built Swift modules because they are explicitly
+  /// built and provided to the compiler invocation.
   bool DisableImplicitModules = false;
 
   /// Disable building Swift modules from textual interfaces. This should be

lib/ClangImporter/ClangImporter.cpp

@@ -669,11 +669,16 @@ importer::getNormalInvocationArguments(
   }
 
   const std::string &moduleCachePath = importerOpts.ModuleCachePath;
-  if (!moduleCachePath.empty()) {
+  if (!moduleCachePath.empty() && !importerOpts.DisableImplicitClangModules) {
     invocationArgStrs.push_back("-fmodules-cache-path=");
     invocationArgStrs.back().append(moduleCachePath);
   }
 
+  if (importerOpts.DisableImplicitClangModules) {
+    invocationArgStrs.push_back("-fno-implicit-modules");
+    invocationArgStrs.push_back("-fno-implicit-module-maps");
+  }
+
   if (ctx.SearchPathOpts.DisableModulesValidateSystemDependencies) {
     invocationArgStrs.push_back("-fno-modules-validate-system-headers");
   } else {

lib/Frontend/CompilerInvocation.cpp

@@ -1460,6 +1460,9 @@ static bool ParseClangImporterArgs(ClangImporterOptions &Opts,
     Opts.PCHDisableValidation |= Args.hasArg(OPT_pch_disable_validation);
   }
 
+  if (FrontendOpts.DisableImplicitModules)
+    Opts.DisableImplicitClangModules = true;
+
   Opts.ValidateModulesOnce |= Args.hasArg(OPT_validate_clang_modules_once);
   if (auto *A = Args.getLastArg(OPT_clang_build_session_file))
     Opts.BuildSessionFilePath = A->getValue();