[clang] fix: constexpr bit_cast with empty base classes (PR #82383)

[via cfe-commits]

sethp wrote:

@cor3ntin @tbaederr is there anything else you need from me here? I would expect github's "squash and merge" button to work fine for this change, but I could rebase if you'd prefer.

https://github.com/llvm/llvm-project/pull/82383