sarif-replay: improve error for unescaped braces in messages (§3.11.5)

Spotted via https://github.com/llvm/llvm-project/issues/128024



gcc/ChangeLog:
	* libsarifreplay.cc
	(sarif_replayer::make_plain_text_within_result_message): Capture
	which json::string was used.  When reporting on unescaped "{" or
	"}" in SARIF message strings, use that string rather than the
	message object, and refer the user to §3.11.5 ("Messages with
	placeholders") rather than §3.11.11 ("arguments").  Ideally we'd
	place the error at the precise character, but that can't be done
	without reworking json-parsing.cc's lexer::lex_string, which is
	too invasive for stage 4.
	(sarif_replayer::get_plain_text_from_mfms): Capture which
	json::string was used.
	(sarif_replayer::lookup_plain_text_within_result_message):
	Likewise.

gcc/testsuite/ChangeLog:
	* sarif-replay.dg/2.1.0-invalid/3.11.11-malformed-placeholder.sarif:
	Rename to...
	* sarif-replay.dg/2.1.0-invalid/3.11.5-unescaped-braces.sarif:
	...this.  Update expected subsection in error message, and
	expected underline in quoted JSON.

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