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

[Fangrui Song via cfe-commits]

MaskRay wrote:

The code checks whether the directory is present `auto CRTPath = TC.getCompilerRTPath();` and only adds the libpath when it is present. This is related to the `LLVM_ENABLE_PER_TARGET_RUNTIME_DIR=off` file hierarchy we are migrating way from.

With `LLVM_ENABLE_PER_TARGET_RUNTIME_DIR=on`, TC.getCompilerRTPath() will not be needed.

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