[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.
> 
> I usually run this under my build directory
> 
> ```
> ./bin/llvm-lit -sv  --param std=c++23  ../libcxx/test/std
> ```
> 
> It no longer works.
> 
> Also I tried to use the `libcxx/utils/libcxx-lit` it failed with the same error.
> 
> But if I revert your commit, everything works again
> 
> I don't use `ninja`. I just used plain `cmake -G "Unix Makefiles"`
> 
> basically I did this https://libcxx.llvm.org/TestingLibcxx.html

I'd need your cmake command line as well to reproduce this.

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