For ivars created using @synthesize, set their DeclContext to be

the @implementation (instead of the @interface) and actually add
the ivar to the DeclContext (which we weren't doing before).

This allows us to simplify ASTContext::CollectNonClassIvars() by
removing ASTContext::CollectProtocolSynthesizedIvars().  Now all
ivars can be found by either inspecting the ObjCInterfaceDecl and
its companion ObjCImplementationDecl.

llvm-svn: 98280