[clang] [clang] add llvm abi support (PR #121123)

[Tristan Ross via cfe-commits]

RossComputerGuy wrote:

Why close? LLVM libc is maturing and one of the goals I had was to try using it in Nixpkgs to build whatever I can. LLVM libc doesn't work well in an overlay mode so we have to use full builds. It's true that a lot of functions are still missing or not implemented but by having the ability to properly specify it can make it easier to find what to do next. Also, from what I can tell Clang and LLVM itself have already gotten some support for the ABI. This PR simply unblocks 1 error which makes the ABI usable.

https://github.com/llvm/llvm-project/pull/121123