[clang] [clang][flang][windows] Prefer user-provided library paths (-L) (PR #90758)

[Andrzej Warzyński via cfe-commits]

banach-space wrote:

> I've left the flang test as the flang directory doesn't change with `LLVM_ENABLE_PER_TARGET_RUNTIME_DIR` and removed the other test. I hope this is what you meant @MaskRay 👍

That's matches how I read that suggestion re `LLVM_ENABLE_PER_TARGET_RUNTIME_DIR`, thanks! 

>From what I can tell, you've already made all the changes requested from @MaskRay . If there are no new comments today, I suggest landing this. 

https://github.com/llvm/llvm-project/pull/90758