[compiler-rt] [runtimes] Allow building against an installed LLVM tree (PR #86209)

[via llvm-commits]

huixie90 wrote:

Hello, After this commit, I can no longer run libc++ tests with llvm-lit anymore.
> fatal: You seem to be running Lit directly -- you should be running Lit through <build>/bin/llvm-lit, which will ensure that the right Lit configuration file is used. See https://libcxx.llvm.org/TestingLibcxx.html#usage for how to run the libc++ tests.

https://github.com/llvm/llvm-project/pull/86209