[clang-format] Fix formatting of _Atomic() qualifier

Before: _Atomic(uint64_t) * a;
After: _Atomic(uint64_t) *a;

This treats _Atomic the same as the the TypenameMacros and decltype. It
also allows some cleanup by removing checks whether the token before a
paren is kw_decltype and instead checking for TT_TypeDeclarationParen.
While touching this code also extend the decltype test cases to also check
for typeof() and _Atomic(T).

Reviewed By: MyDeveloperDay

Differential Revision: https://reviews.llvm.org/D86959