diagnostics: JSON output: use std::unique_ptr throughout

diagnostics: JSON output: use std::unique_ptr throughout

No functional change intended.

gcc/ChangeLog:
	* diagnostic-format-json.cc: Include "make-unique.h".
	(json_output_format::m_toplevel_array): Convert to
	std::unique_ptr.
	(json_output_format::json_output_format): Update accordingly.
	(json_output_format::~json_output_format): Remove manual
	"delete" of field.
	(json_from_expanded_location): Convert return type to
	std::unique_ptr.
	(json_from_location_range): Likewise.  Use nullptr rather than
	NULL.
	(json_from_fixit_hint): Convert return type to std::unique_ptr.
	(json_from_metadata): Likewise.
	(make_json_for_path): Likewise.
	(json_output_format::on_end_diagnostic): Use std::unique_ptr
	throughout.
	(json_file_output_format::~json_file_output_format): Use nullptr.
	(selftest::test_unknown_location): Update to use std::unique_ptr.
	(selftest::test_bad_endpoints): Likewise.  Replace NULL with
	nullptr.

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