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

[Timm Baeder via cfe-commits]

tbaederr wrote:

> This will need a release note. I'm curious why the removed code was there in the first place. Any idea @tbaederr ?

No idea, it's been there since the original commit introducing `__builtin_bit_cast`. Maybe just an optimization, or maybe something else in that loop used to assert that the class isn't empty.

Aside from the release not, this LGTM.

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