[clang] Patch series to reapply #118734 and substantially improve it (PR #120534)

[Chandler Carruth via cfe-commits]

https://github.com/chandlerc commented:

I think I've addressed most of the review comments here at this point.

But maybe most excitingly, I think the latest version may dodge the issues that have cropped up with MSVC -- both LoongArch and X86 fixes have been incorporated that hopefully help. @dyung -- if you can try it out, let me know.

There are three PRs factored out of this and under independent review:
- #120831
- #120835 (response to review comment)
- #120861

The patches that are still in this series because I don't really know how to send them out independently without breaking MSVC have some changes from the initial PR going out:

- Now sharding LoongArch as well as other targets in the PR that introduces sharding
- An extra commit in the series specifically moving the main builtins and several targets that use the same tablegen emission logic to not use X-macros as much. This is a big new commit and should get specific review in this PR before it lands: [`fcb7395` (#120534)](https://github.com/llvm/llvm-project/pull/120534/commits/fcb739514d298a2cf0b33d4dacfa2a2ce7277c68)

Beyond this, see responses to code reviwe comments inline.

https://github.com/llvm/llvm-project/pull/120534