PR14772: Support constant expression evaluation for _Atomic types.

 * Treat _Atomic(T) as a literal type if T is a literal type.
 * Evaluate expressions of this type properly.
 * Fix a lurking bug where we built completely bogus ASTs for converting to
   _Atomic types in C++ in some cases, caught by the tests for this change.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@182541 91177308-0d34-0410-b5e6-96231b3b80d8