diagnostics: tweak output for nested messages [PR116253]

diagnostics: tweak output for nested messages [PR116253]

When printing nested messages with
-fdiagnostics-set-output=text:experimental-nesting=yes
avoid printing a line such as the "cc1plus:" in the following:
        • note: set ‘-fconcepts-diagnostics-depth=’ to at least 2 for more detail
          cc1plus:
for "special" locations such as UNKNOWN_LOCATION.

gcc/ChangeLog:
	PR other/116253
	* diagnostic-format-text.cc (on_report_diagnostic): When showing
	locations for nested messages on new lines, don't print
	UNKNOWN_LOCATION or BUILTINS_LOCATION.

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