[libc++] Avoids self references in transitive include test.

The output of --trace-includes starts with the header whose includes are
being processed. Since the sanitize script processed all lines this
include was added to the list of transitive includes. This looks odd
since it implies all headers have a cyclic dependency on themselves.
This change removes this self-include.

Instead of just dropping the first line extract that header and use it
to guard against cyclic dependencies in the header itself.

The regex used has a small improvement; don't capture groups that aren't
extracted.

Depends on D132284

Reviewed By: ldionne, #libc

Differential Revision: https://reviews.llvm.org/D132787