[clang] [Clang][C++20] Implement constexpr std::bit_cast for bit-fields (& [Sema] Print more static_assert exprs) (PR #74775)

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sethp wrote:

Easy enough: I opened a PR for just those changes as https://github.com/llvm/llvm-project/pull/74852 . For now, I hope it's OK if I leave them here too as I continue working on the bit-cast part: I'd still like to have them locally, and there's a lot of `git rebase -i` involved to make that happen otherwise.

https://github.com/llvm/llvm-project/pull/74775