[Clang][Sema] Avoid crashing for va_arg expressions with bool argument

This change fixes a compiler crash that was introduced in https://reviews.llvm.org/D103611: `Sema::BuildVAArgExpr` attempted to retrieve a corresponding signed type for `bool` by calling `ASTContext::getCorrespondingSignedType`.

rdar://86580370

Reviewed By: aaron.ballman

Differential Revision: https://reviews.llvm.org/D116272