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

[Alexander Richardson via llvm-commits]

https://github.com/arichardson approved this pull request.

This looks fine to me, although the setting of `LLVM_EXTERNAL_LIT_MISSING_WARNED_ONCE` is incorrect if LLVM_EXTERNAL_LIT` was already set when entering this file. Probably doesn't matter since people who manually set `LLVM_EXTERNAL_LIT` probably know what they are doing and if it is set to something invalid, then we just fail a bit later in the configure process instead.

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