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

[Tristan Ross via cfe-commits]

RossComputerGuy wrote:

> That said, I tend to agree with the review at [#121123 (comment)](https://github.com/llvm/llvm-project/pull/121123#discussion_r1900548165) that this doesn't seem like the right approach.

Yeah I agree too, I'm not sure what the right approach is but that's one of the reasons why PR's get reviewed.

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