Instead of modifying the ObjC AST to not modify existing declarations, teach chained...

Instead of modifying the ObjC AST to not modify existing declarations, teach chained PCH to overwrite declarations from earlier PCH files in dependent ones. Tell Sema to note when it changes AST nodes so that they have to be reserialized. Finally, the ObjCProtocolDecls created in forward decls, like the ObjCInterfaceDecls in @class forward decls, are not lexically part of the decl context; only the definition is.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@110989 91177308-0d34-0410-b5e6-96231b3b80d8