[clang] [Driver] Improve error when a compiler-rt library is not found (PR #81037)

[Martin Storsjö via cfe-commits]

https://github.com/mstorsjo approved this pull request.

LGTM, thanks.

In principle, we're just trading surprises in one case into surprises in another case, but I guess this case is the one with more non-obvious details (and this can be argued is the future direction we should be heading towards, even if I'm not personally a huge fan of it), so it seems reasonable and valuable.

https://github.com/llvm/llvm-project/pull/81037