[clang] [AMDGPU] Programmatically port old `.def` clang builtins to `.td` (PR #175873)

[Joseph Huber via cfe-commits]

jhuber6 wrote:

> @jhuber6 you've said what this is supposed to do but not why, and not why now. Please explain.

The TableGen format is the newer format that targets are slowly porting to, it just makes adding new features easier. This is an NFC change to modernize it more or less.

https://github.com/llvm/llvm-project/pull/175873