[clang] [clang] Introduce a new UEFI target predefine. (PR #111719)

[Joseph Huber via cfe-commits]

jhuber6 wrote:

Didn't know this existed, but we have https://github.com/llvm/llvm-project/pull/120632 tracking it now.

https://github.com/llvm/llvm-project/pull/111719