[libcxx-commits] [libcxx] [libc++][utils] Fix LIT `%if` substitution support (PR #209358)

[Hubert Tong via libcxx-commits]

hubert-reinterpretcast wrote:

There is no reason to believe that the failure in "[Build and Test libc++ / stage3 (bootstrapping-build)](https://github.com/llvm/llvm-project/actions/runs/29341201833/job/87126499928?pr=209358)" on `lldb-api :: lang/cpp/std-function-step-into-callable/TestStdFunctionStepIntoCallable.py` has anything to do with the change here.

https://github.com/llvm/llvm-project/pull/209358