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

[Martin Storsjö via cfe-commits]

mstorsjo wrote:

I would, generally, prefer to not hardcode `LLVM_ENABLE_PER_TARGET_RUNTIME_DIR` (which only affects how runtimes are installed) into Clang. Runtimes may or may not be built at the same time as Clang, and one build of Clang can be used for a multitude of targets with different setups. I wrote a more lengthy response on discourse, see https://discourse.llvm.org/t/runtime-directory-fallback/76860/7.

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