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

[Alexander Richardson via llvm-commits]

arichardson 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 /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.

Can you send me the commands you are running? Also let me know what `ninja check-all -v -n` prints.

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