nullptr returned from ActOnTag() is not a valid result

DeclResult tracks two states: valid/invalid and usable/unusable.
Passing a null pointer to the constructor creates a valid but unusable
result and we wanted an invalid result instead. This changes some
functions to return a DeclResult rather than a Decl * to make it harder
to get this incorrect in callers.

Discovered when working on https://reviews.llvm.org/D141280

.

Co-authored-by: Haojian Wu <hokein@google.com>
Co-authored-by: Aaron Ballman <aaron@aaronballman.com>

Differential Revision: https://reviews.llvm.org/D141580