Sema: adjust assertion to account for deduced types

Previous changes for the BTF attributes introduced a new sub-tree
visitation.  That uncovered that when accessing the typespec location we
would assert that the type specification is either a type declaration or
`typename`.  However, `typename` was explicitly permitted.  This change
predates the introduction of newer deduced type representations such as
`__underlying_type` from C++ and the addition of the GNU `__typeof__`
expression.

Thanks to aaron.ballman for the valuable discussion and pointer to
`isTypeRep`.

Differential Revision: https://reviews.llvm.org/D126093
Reviewed By: aaron.ballman, yonghong-song