Make parsing of objc @implementations more robust.

Parsing of @implementations was based on modifying global state from
the parser; the logic for late parsing of methods was spread in multiple places
making it difficult to have a robust error recovery.

  -it was difficult to ensure that we don't neglect parsing the lexed methods.
  -it was difficult to setup the original objc container context for parsing the lexed methods
   after completing ParseObjCAtImplementationDeclaration and returning to top level context.

Enhance parsing of @implementations by centralizing it in Parser::ParseObjCAtImplementationDeclaration().
ParseObjCAtImplementationDeclaration now returns only after an @implementation is fully parsed;
all the data and logic for late parsing of methods is now in one place.

This allows us to provide code-completion for late parsed methods with mis-matched braces.
rdar://10775381

llvm-svn: 149987