[clang] [clang] Ensure `--print-runtime-dir` path exists (PR #102834)

[Alexandre Ganea via cfe-commits]

aganea wrote:

Ping.

This still occurs on Windows as of today on main, `clang-cl -print-runtime-dir` doesn't print the right folder path.
On disk we have: `C:\Program Files\LLVM\lib\clang\22\lib\windows`,
whereas `-print-runtime-path` prints the full target as on the other platforms: `C:\Program Files\LLVM\lib\clang\22\lib\x86_64-pc-windows-msvc`.

What is it that we want to do here? Add a dynamic behavior as in this PR, that checks if the path is there, and adjusts accordingly?Change the "windows" component to the target, as the other plateforms (this seems to be some work). Or conversly, accept that on Windows the path is "windows" and not the target? Anything else?

https://github.com/llvm/llvm-project/pull/102834