sarif-replay: handle embedded links (§3.11.6)

Handle embedded links in plain text messages.  For now, merely
use the link text and discard the destination.

gcc/ChangeLog:
	* libsarifreplay.cc (struct embedded_link): New.
	(maybe_consume_embedded_link): New.
	(sarif_replayer::make_plain_text_within_result_message): Handle
	embedded links by using the link text, for now.

gcc/testsuite/ChangeLog:
	* sarif-replay.dg/2.1.0-valid/3.11.6-embedded-links.sarif: New test.
	* sarif-replay.dg/2.1.0-valid/malloc-vs-local-4.c.sarif: Update
	expected output for handling the embedded links.
	* sarif-replay.dg/2.1.0-valid/spec-example-4.sarif: Likewise.

Signed-off-by: David Malcolm <dmalcolm@redhat.com>