[clang] [llvm] [clang-tools-extra] [clang] Refactor Builtins.def to be a tablegen file (PR #68324)

[Aaron Ballman via cfe-commits]

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

The changes LGTM, thank you for this awesome refactoring! Thank you!

Please wait to land until after we branch for Clang 18 and after CI has finish running (it'll be a race to see which happens first).

https://github.com/llvm/llvm-project/pull/68324