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

[Fangrui Song via cfe-commits]

MaskRay wrote:

NAK. While llvm libc can be used an overlay, it is not complete for standalone Linux server/desktop usage. Introducing a target triple has large ecosystem implication and should not be taken lightly. 

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