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

[Vitaly Buka via cfe-commits]

vitalybuka wrote:

> Also https://lab.llvm.org/buildbot/#/builders/66/builds/1427/steps/13/logs/stdio
> 
> ```
> [20/21] Running compiler_rt regression tests
> llvm-lit: /b/sanitizer-x86_64-linux/build/llvm-project/compiler-rt/test/lit.common.cfg.py:60: warning: Path reported by clang does not exist: "/b/sanitizer-x86_64-linux/build/build_debug/lib/clang/19/lib/i386-unknown-linux-gnu". This path was found by running ['/b/sanitizer-x86_64-linux/build/build_debug/./bin/clang', '--target=x86_64-unknown-linux-gnu', '-m32', '-print-runtime-dir'].
> llvm-lit: /b/sanitizer-x86_64-linux/build/llvm-project/compiler-rt/test/lit.common.cfg.py:60: warning: Path reported by clang does not exist: "/b/sanitizer-x86_64-linux/build/build_debug/lib/clang/19/lib/i386-unknown-linux-gnu". This path was found by running ['/b/sanitizer-x86_64-linux/build/build_debug/./bin/clang', '--target=x86_64-unknown-linux-gnu', '-m32', '-print-runtime-dir'].
> llvm-lit: /b/sanitizer-x86_64-linux/build/llvm-project/compiler-rt/test/lit.common.cfg.py:60: warning: Path reported by clang does not exist: "/b/sanitizer-x86_64-linux/build/build_debug/lib/clang/19/lib/x86_64-unknown-linux-gnu". This path was found by running ['/b/sanitizer-x86_64-linux/build/build_debug/./bin/clang', '--target=x86_64-unknown-linux-gnu', '-m64', '-print-runtime-dir'].
> llvm-lit: /b/sanitizer-x86_64-linux/build/llvm-project/compiler-rt/test/lit.common.cfg.py:60: warning: Path reported by clang does not exist: "/b/sanitizer-x86_64-linux/build/build_debug/lib/clang/19/lib/x86_64-unknown-linux-gnu". This path was found by running ['/b/sanitizer-x86_64-linux/build/build_debug/./bin/clang', '--target=x86_64-unknown-linux-gnu', '-m64', '-print-runtime-dir'].
> llvm-lit: /b/sanitizer-x86_64-linux/build/llvm-project/compiler-rt/test/lit.common.cfg.py:60: warning: Path reported by clang does not exist: "/b/sanitizer-x86_64-linux/build/build_debug/lib/clang/19/lib/x86_64-unknown-linux-gnu". This path was found by running ['/b/sanitizer-x86_64-linux/build/build_debug/./bin/clang', '--target=x86_64-unknown-linux-gnu', '-m64', '-print-runtime-dir'].
> ```

Looks like it caused by use on the bots of `-DLLVM_ENABLE_PER_TARGET_RUNTIME_DIR=OFF`

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