[clang] [Driver] Ensure ToolChain::LibraryPaths is not empty for non-Darwin (PR #87866)

[Martin Storsjö via cfe-commits]

mstorsjo wrote:

This seems to have had an unexpected effect. In a build where I don't use the new path style, I used to get the old path style returned like this:
```
$ clang -target x86_64-w64-mingw32 -print-runtime-dir
/home/martin/clang-nightly/lib/clang/19/lib/windows
```
However after this change, now I'm getting the new style path, even if it doesn't exist, and if the old one actually did exist:
```
$ clang -target x86_64-w64-mingw32 -print-runtime-dir
/home/martin/clang-nignhtly/lib/clang/19/lib/x86_64-w64-windows-gnu
```
I'm ok with changing the default if the old path style doesn't exist - but if it does exist, we should still return that. (I haven't dig into it to see why this is, yet.)

https://github.com/llvm/llvm-project/pull/87866