[clang] [clang][NFC] Refactor Builtins.def to be a tablegen file (PR #68324)

[Aaron Ballman via cfe-commits]

https://github.com/AaronBallman commented:

My review may take a bit because GitHub is struggling to render the changes in Builtins.td.

However, I want to state up front just how incredibly excited I am for this refactoring. This will go a long ways towards letting us start to generate documentation for all of our builtins instead of manually having to add it to the LanguageExtensions.rst file (which is easy for folks to forget or ignore). So despite the extra complexity of using .td file, I think it's worth the complexity. Thank you for the work!

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