diagnostics: fix memory leak in SARIF selftests

"make selftest-valgrind" was complaining about leaks of artifact objects
in SARIF's selftest::test_make_location_object:

-fself-test: 7638695 pass(es) in 89.999249 seconds
==3306525==
==3306525== HEAP SUMMARY:
==3306525==     in use at exit: 1,215,639 bytes in 2,808 blocks
==3306525==   total heap usage: 2,860,898 allocs, 2,858,090 frees, 1,336,446,579 bytes allocated
==3306525==
==3306525== 11,728 (1,536 direct, 10,192 indirect) bytes in 16 blocks are definitely lost in loss record 353 of 375
==3306525==    at 0x514FE7D: operator new(unsigned long) (vg_replace_malloc.c:342)
==3306525==    by 0x36E5FD2: sarif_builder::get_or_create_artifact(char const*, diagnostic_artifact_role, bool) (diagnostic-format-sarif.cc:2884)
==3306525==    by 0x36E3D57: sarif_builder::maybe_make_physical_location_object(unsigned int, diagnostic_artifact_role, int, content_renderer const*) (diagnostic-format-sarif.cc:2097)
==3306525==    by 0x36E34CE: sarif_builder::make_location_object(sarif_location_manager&, rich_location const&, logical_location const*, diagnostic_artifact_role) (diagnostic-format-sarif.cc:1922)
==3306525==    by 0x36E72C6: selftest::test_make_location_object(selftest::line_table_case const&) (diagnostic-format-sarif.cc:3500)
==3306525==    by 0x375609B: selftest::for_each_line_table_case(void (*)(selftest::line_table_case const&)) (input.cc:3898)
==3306525==    by 0x36E9668: selftest::diagnostic_format_sarif_cc_tests() (diagnostic-format-sarif.cc:3910)
==3306525==    by 0x3592A11: selftest::run_tests() (selftest-run-tests.cc:100)
==3306525==    by 0x17DBEF3: toplev::run_self_tests() (toplev.cc:2268)
==3306525==    by 0x17DC2BF: toplev::main(int, char**) (toplev.cc:2376)
==3306525==    by 0x36A1919: main (main.cc:39)
==3306525==
==3306525== 12,400 (1,536 direct, 10,864 indirect) bytes in 16 blocks are definitely lost in loss record 355 of 375
==3306525==    at 0x514FE7D: operator new(unsigned long) (vg_replace_malloc.c:342)
==3306525==    by 0x36E5FD2: sarif_builder::get_or_create_artifact(char const*, diagnostic_artifact_role, bool) (diagnostic-format-sarif.cc:2884)
==3306525==    by 0x36E2323: sarif_builder::sarif_builder(diagnostic_context&, line_maps const*, char const*, bool) (diagnostic-format-sarif.cc:1500)
==3306525==    by 0x36E70AA: selftest::test_make_location_object(selftest::line_table_case const&) (diagnostic-format-sarif.cc:3469)
==3306525==    by 0x375609B: selftest::for_each_line_table_case(void (*)(selftest::line_table_case const&)) (input.cc:3898)
==3306525==    by 0x36E9668: selftest::diagnostic_format_sarif_cc_tests() (diagnostic-format-sarif.cc:3910)
==3306525==    by 0x3592A11: selftest::run_tests() (selftest-run-tests.cc:100)
==3306525==    by 0x17DBEF3: toplev::run_self_tests() (toplev.cc:2268)
==3306525==    by 0x17DC2BF: toplev::main(int, char**) (toplev.cc:2376)
==3306525==    by 0x36A1919: main (main.cc:39)
==3306525==
==3306525== LEAK SUMMARY:
==3306525==    definitely lost: 3,072 bytes in 32 blocks
==3306525==    indirectly lost: 21,056 bytes in 368 blocks
==3306525==      possibly lost: 0 bytes in 0 blocks
==3306525==    still reachable: 1,191,511 bytes in 2,408 blocks
==3306525==         suppressed: 0 bytes in 0 blocks
==3306525== Reachable blocks (those to which a pointer was found) are not shown.
==3306525== To see them, rerun with: --leak-check=full --show-leak-kinds=all
==3306525==
==3306525== For lists of detected and suppressed errors, rerun with: -s
==3306525== ERROR SUMMARY: 2 errors from 2 contexts (suppressed: 0 from 0)

Fixed thusly.

gcc/ChangeLog:
	* diagnostic-format-sarif.cc (sarif_builder::~sarif_builder): New,
	deleting any remaining artifact objects.
	(sarif_builder::make_run_object): Empty the artifact map.
	* ordered-hash-map.h (ordered_hash_map::empty): New.

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