Fix ICE in get_substring_ranges_for_loc on __FILE__ (PR c++/87721)

Fix ICE in get_substring_ranges_for_loc on __FILE__ (PR c++/87721)

PR c++/87721 reports a crash in get_substring_ranges_for_loc introduced
by r265271, my fix for PR 87562.

The new issue occurs when attempting to get a location with a string
literal inside a macro in which the first token is __FILE__ (formed via
concatenation).  Attempting to get the spelling location of __FILE__
fails, leading to NULL for start_ord_map and final_ord_map, and thus
a NULL pointer dereference.

Given that our "on-demand" substring locations approach reparses the
string literals, there isn't a good way to access the locations inside
such string literals: attempting to reparse __FILE__ fails with a
"missing open quote".

This patch applies the easy fix by gracefully rejecting the case where
the spelling locations for the start or finish give us NULL maps.

gcc/ChangeLog:
	PR c++/87721
	* input.c (get_substring_ranges_for_loc): Detect if
	linemap_resolve_location gives us a NULL map, and reject
	this case.

gcc/testsuite/ChangeLog:
	PR c++/87721
	* c-c++-common/substring-location-PR-87721.c: New test.
	* gcc.dg/plugin/diagnostic-test-string-literals-1.c: Add test for
	PR 87721.
	* gcc.dg/plugin/diagnostic_plugin_test_string_literals.c
	(test_string_literals): Fold the index arguments before checking
	for INTEGER_CST.

From-SVN: r265611