[llvm] [Support] Better error msg when cache dir can't be created. (PR #69575)

[Teresa Johnson via llvm-commits]

https://github.com/teresajohnson commented:

Change looks fine but can you add a test? Looks like unfortunately the other error conditions here are not tested anywhere, but let's just start by adding one for the error modified here. There are some existing caching tests in llvm/test/ThinLTO/X86/ (look for the cache-dir flag) that can be used as examples.

https://github.com/llvm/llvm-project/pull/69575