[clang] [Clang][C++20] Implement constexpr std::bit_cast for bit-fields (PR #74775)

[Seth Pellegrino via cfe-commits]

sethp wrote:

@eisenwave I am sorry to report that I am not going to have time to continue to work on this PR for the foreseeable future. As I understand it, the two blockers are incorporating feedback into https://github.com/llvm/llvm-project/pull/74852  (on which this change depends) and then a review of the changes here.

If I can support either of those tasks with answers to questions about what's in here or in that other PR, I'll do my best to provide context.

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