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

[Tobias Hieta via llvm-commits]

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+; REQUIRES: x86
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tru wrote:

My first iteration was there - but it doesn't seem to follow the same code path, I never got the error message that I got from lld-link.

I will merge this now, since a test is better than no test - but I might follow up with a test in the llvm layer when I do the changes to report_fatal_error if people think that's a good change.

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