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

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.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@98280 91177308-0d34-0410-b5e6-96231b3b80d8