[clang] [clang][Interp] Implement __builtin_bitreverse (PR #71687)

[Aaron Ballman via cfe-commits]

https://github.com/AaronBallman approved this pull request.

The changes seem reasonable to me but I think we should start putting some effort in to support _BitInt because I have a sneaking suspicion that will be hard to support with all the power-of-two literals being used, and the more integer handling code we add, the harder it will be to support in the future. WDYT?

https://github.com/llvm/llvm-project/pull/71687