[clang][AST] Print name instead of type when diagnosing uninitialized...

[clang][AST] Print name instead of type when diagnosing uninitialized subobject in constexpr variables

This patch improves the diagnostic on uninitialized subobjects in constexpr variables by modifying the diagnostic message to display the subobject's name instead of its type.

Fixes https://github.com/llvm/llvm-project/issues/58601
Differential Revision: https://reviews.llvm.org/D146358