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

[via cfe-commits]

sethp wrote:

Just to cross-link, I originally asked about this on discord: https://discord.com/channels/636084430946959380/636732781086638081/1209534556126973993 

(cc @cor3ntin )

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