[clang] [Clang] Start moving X86Builtins.def to X86Builtins.td (PR #106005)

[Nikolas Klauser via cfe-commits]

philnik777 wrote:

> @philnik777 Is this part of a larger scheme covering other targets?

Ultimately all the builtins should be in TableGen files instead of the `.def` files, but that's going to take a while. Probably years, since it's not exactly high priority.

https://github.com/llvm/llvm-project/pull/106005