Sema: Handle declspecs without declarators in records properly in C mode

We had two bugs:
- We wouldn't properly warn when a struct/union/enum was mentioned
  inside of a record definition if no declarator was provided.  We
  should have mentioned that this declaration declares nothing.
- We didn't properly support Microsoft's extension where certain
  declspecs without declarators would act as anonymous structs/unions.
  * We completely ignored the case where such a declspec could be a
    union.
  * We didn't properly handle the case where a record was defined inside
    another record:
      struct X {
        int a;
        struct Y {
          int b;
        };
      };