[Driver,Darwin] Simplify libc++ include path after #80527. NFC

My #80527 mentioned that `InstalledDir` was weird in the
-canonical-prefixes mode. #70817 was a workaround to find the libc++
include path for a symlinked clang. After #80527, `InstalledDir` was
identical to `Dir` and was subsequently removed, the code change #70817
can be reverted.