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

[Simon Pilgrim via cfe-commits]

RKSimon wrote:

No objections to this patch, I do want to see other targets moved over as well though, instead of x86 ending up being treated as a special/weird case.

I am preparing to convert a lot of x86 builtins to be constexpr (#30794) - but as long as this PR won't interfere I see no big reason not to get it done right away.

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