Return a declaration to the parser when creating a field in C++ so that

Return a declaration to the parser when creating a field in C++ so that
the parser will complete the declarator with a valid decl and thus trigger
delayed diagnostics for it.  It certainly looks like we were intentionally
returning null here, but I couldn't find any good reason for it, and there
wasn't a comment, so farewell to all that.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@125556 91177308-0d34-0410-b5e6-96231b3b80d8