[ParseableInterface] Turn on -enable-parseable-module-interface always (#23331)

...and remove the option. This is ~technically~ CLI-breaking because
Swift 5 shipped this as a hidden driver option, but it wouldn't have
/done/ anything in Swift 5, so I think it's okay to remove.

Note that if a parseable interface (.swiftinterface) and a binary
interface (.swiftmodule) are both present, the binary one will still
be preferred. This just /allows/ parseable interfaces to be used.

rdar://problem/36885834

tools/SourceKit/lib/SwiftLang/SwiftEditorInterfaceGen.cpp

@@ -708,7 +708,6 @@ void SwiftLangSupport::editorOpenInterface(EditorConsumer &Consumer,
   }
 
   Invocation.getClangImporterOptions().ImportForwardDeclarations = true;
-  Invocation.getFrontendOptions().EnableParseableModuleInterface = true;
 
   std::string ErrMsg;
   auto IFaceGenRef = SwiftInterfaceGenContext::create(Name,