Remove API notes compilation support from swiftc (#21759)

We don't use this feature anymore (see 0149129), and now that we've
branched Swift 5.0 for the last time it's time to rip it out of swiftc
(and then from Clang).

No functionality change, unless someone was compiling their own API
notes. If you were doing that, just copy them into place as text.

Author: jrose-apple

test/APINotes/Inputs/roundtrip.apinotes

@@ -1,5 +1,4 @@
 add_swift_host_tool(swift
-  api_notes.cpp
   driver.cpp
   autolink_extract_main.cpp
   modulewrap_main.cpp

test/APINotes/yaml-roundtrip.swift

@@ -111,8 +111,6 @@ static bool shouldRunAsSubcommand(StringRef ExecName,
   return true;
 }
 
-extern int apinotes_main(ArrayRef<const char *> Args);
-
 static int run_driver(StringRef ExecName,
                        const ArrayRef<const char *> argv) {
   // Handle integrated tools.
@@ -128,10 +126,6 @@ static int run_driver(StringRef ExecName,
                                                 argv.data()+argv.size()),
                              argv[0], (void *)(intptr_t)getExecutablePath);
     }
-    if (FirstArg == "-apinotes") {
-      return apinotes_main(llvm::makeArrayRef(argv.data()+1,
-                                              argv.data()+argv.size()));
-    }
   }
 
   std::string Path = getExecutablePath(argv[0]);