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

[Fangrui Song via cfe-commits]

MaskRay wrote:

I added an AIX workaround because AIX hasn't migrated to the new layout and two AIX tests would fail without the workaround. 

@daltenty 

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