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

[Anton Korobeynikov via cfe-commits]

asl wrote:

@sam-mccall As far as I can see, the bot was green before this change: https://lab.llvm.org/buildbot/#/builders/119/builds/15229

Have you not received email from the buildbot?

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