[clang] [clang] Fix C23 constexpr crashes (PR #112708)

[Timm Baeder via cfe-commits]

tbaederr wrote:

Agreed, LGTM. The first comment in `VarDecl::hasConstantInitialization()` could use some love, since it's no longer true for global constexpr variables.

https://github.com/llvm/llvm-project/pull/112708