PR c/70264: fix crash in compatible_locations_p with BUILTINS_LOCATION

PR c/70264: fix crash in compatible_locations_p with BUILTINS_LOCATION

In r234088 my fix for PR c++/70105 didn't allow for the possibility that
when comparing a pair of macro expansion histories that one of the
macros in the history might not be located within a line-map, and
PR c/70264 reports a crash due to encountering BUILTINS_LOCATION within
the traversal.

Fixed thusly.

Successfully bootstrapped on x86_64-pc-linux-gnu; adds 4 PASS results
to gcc.sum and 12 to g++.sum.

gcc/ChangeLog:
	PR c/70264
	* diagnostic-show-locus.c (compatible_locations_p): Handle the case
	where one or both locations aren't within a line_map.

gcc/testsuite/ChangeLog:
	PR c/70264
	* c-c++-common/pr70264.c: New test case.

From-SVN: r234303