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

[Sam McCall via cfe-commits]

sam-mccall wrote:

Sorry, there were no notifications, I guess the bot was already red.

I'm not really sure what to do other than disable the test on these bots - I don't know why it would fail on win/arm (it seems to pass on win/x86), and I don't have access to such a machine.
This behavior isn't particularly relevant on windows but is critical on linux. This isn't a regression, there was just previously no testing of it.

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