[Clang] Adjust triviality computation in QualType::isTrivialType to C++20 cases.

Up to C++20, hasDefaultConstructor and !hasNonTrivialDefaultConstructor together implied
hasTrivialDefaultConstructor. In C++20, a constructor might be ineligible and can set
hasDefaultConstructor without setting hasNonTrivialDefaultConstructor.

Fix this by querying hasTrivialDefaultConstructor instead of hasDefaultConstructor.

I'd like to backport this to Clang 16.
I only change isTrivialType and in a way that should only affect code that uses
constrained constructors, so I think this is relatively safe to backport.

Fixes https://github.com/llvm/llvm-project/issues/60697

Reviewed By: aaron.ballman

Differential Revision: https://reviews.llvm.org/D143891