PR c++/90462 reports an ICE with -fdiagnostics-format=json when
attempting to serialize a malformed location to JSON.

The compound location_t in question has meaningful "caret" and "start"
locations, but has UNKNOWN_LOCATION for its "finish" location,
leading to a NULL pointer dereference when attempting to build a JSON
string for the filename.

This patch bulletproofs the JSON output so that attempts to write
a JSON object for a location with a NULL file will lead to an object
with no "file" key, and attempts to write a compound location with
UNKNOWN_LOCATION for its start or finish will lead to the corresponding
JSON child object being omitted.

This patch also adds a json::object::get member function, for self-testing
the above.

gcc/ChangeLog:
	PR c++/90462
	* diagnostic-format-json.cc: Include "selftest.h".
	(json_from_expanded_location): Only add "file" key for non-NULL
	file strings.
	(json_from_location_range): Don't add "start" and "finish"
	children if they are UNKNOWN_LOCATION.
	(selftest::test_unknown_location): New selftest.
	(selftest::test_bad_endpoints): New selftest.
	(selftest::diagnostic_format_json_cc_tests): New function.
	* json.cc (json::object::get): New function.
	(selftest::test_object_get): New selftest.
	(selftest::json_cc_tests): Call it.
	* json.h (json::object::get): New decl.
	* selftest-run-tests.c (selftest::run_tests): Call
	selftest::diagnostic_format_json_cc_tests.
	* selftest.h (selftest::diagnostic_format_json_cc_tests): New
	decl.

gcc/testsuite/ChangeLog:
	PR c++/90462
	* g++.dg/pr90462.C: New test.

From-SVN: r271535