The existing code incorrectly assumes that `Path` can be empty. It
can't, it always contains at least `<` or `"`. On Unix, this patch fixes
an incorrect diagnostics that instead of `"/Users/blah"` suggested
`"Userss/blah"`. In assert builds, this would outright crash.
This patch also fixes a bug on Windows that would prevent the diagnostic
being triggered due to separator mismatch.
rdar://91172342
f0691bcdf9