[llvm] [runtimes] Allow use of external llvm-lit on standalone builds (PR #144347)

[Alexander Richardson via llvm-commits]

arichardson wrote:

> Thanks for the review!
> 
> When `ALLOW_EXTERNAL` is used, the `get_llvm_lit_path` function itself will check if the path provided exists and issue the warning, so I believe it's still ok the set `LLVM_EXTERNAL_LIT_MISSING_WARNED_ONCE` after that call:
> 
> https://github.com/llvm/llvm-project/blob/64bd4858dc2d64311622e793b66094b07ca7bdc5/llvm/cmake/modules/AddLLVM.cmake#L2039-L2050

Ah yes, sounds good to me then :)

https://github.com/llvm/llvm-project/pull/144347