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

[Timm Baeder via cfe-commits]

tbaederr wrote:

I don't have the capacity to review this properly, but the changes to the `static_assert` diagnostics should be split out IMO.

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