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

[Sam McCall via cfe-commits]

sam-mccall wrote:

... and as soon as I said that, I found something: looks like these are maybe building with `DEFAULT_SYSROOT=C:/buildbot/.arm-ubuntu`, which breaks the no-sysroot behavior we're testing here.

I'll see if I can override with `--sysroot=`

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