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

[hazohelet]

Summary

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

[shafik]

Thank you for submitting this fix.

clang/lib/AST/ExprConstant.cpp

2270

To conform to clang-tidy bugprone-argument-comment format

2399

same here.

2444

Also here

2456

and here

clang/lib/AST/Interp/Interp.cpp

373

Do we need to check that SubObjDecl is not nullptr ever?

[shafik]

clang/lib/AST/Interp/Interp.cpp

373

If we are sure it can never be nullptr then we should assert(SubObjDecl)

[tbaeder]

"subobject named 'foo'" sounds a bit weird to me, I'd expect just "subobject 'foo'", but that's just a suggestion and I'll wait for a native spearker to chime in on this.

clang/lib/AST/Interp/Interp.cpp

373

Should never be null here.

[hazohelet]

Address comments from @shafik and @tbaeder