[clang] [Driver] Fix detection of libc++ with empty sysroot. (PR #66947)

[Vladimir Vereschaka via cfe-commits]

vvereschaka wrote:

@sam-mccall ,
your 7ca8c21af36acb117529e797b3d36e85a286ca47 commit breaks `Clang-Unit :: Driver/./ClangDriverTests.exe/ToolChainTest/VFSGnuLibcxxPathNoSysroot` (`Clang-Unit::67`) test on the windows toolchain builders
* https://lab.llvm.org/buildbot/#/builders/119/builds/15230
* https://lab.llvm.org/buildbot/#/builders/60/builds/14031 

The problem remains for more than 3 days already. Would you fix it as soon as it possible or revert your changes?


https://github.com/llvm/llvm-project/pull/66947