diagnostics::output_spec: support client-specific keys

diagnostics::output_spec: support client-specific keys

This patch adds the ability for users of diagnostics::output_spec to
add custom key/values pairs to
  -fdiagnostics-{add,set}-output=SCHEME:KEY=VALUE

The intent is for allow e.g. capturing compiler-specific information in
SARIF sinks (CFGs, inheritance hierarchies, etc)

gcc/ChangeLog:
	* diagnostics/output-spec.cc: (scheme_handler::parse_bool_value):
	Convert to...
	(key_handler::parse_bool_value): ...this.
	(scheme_handler::parse_enum_value): Convert to...
	(key_handler::parse_enum_value): ...this.
	(struct text_scheme_handler::decoded_args): Eliminate, moving
	fields into class text_scheme_handler.
	(text_scheme_handler::text_scheme_handler): Initialize the new
	fields.
	Add a "dc" param and use it to initialize m_show_color.
	(struct sarif_scheme_handler::decoded_args): Eliminate, moving
	fields into class sarif_scheme_handler.
	(sarif_scheme_handler::sarif_scheme_handler): Initialize the new
	fields.
	(struct html_scheme_handler::decoded_args): Eliminate, moving
	fields into class html_scheme_handler.
	(html_scheme_handler::html_scheme_handler): Initialize the new
	fields.
	(context::report_unknown_key): Get keys from scheme rather than
	passing them in.  Support client keys.
	(context::parse_and_make_sink): Pass dc to output_factory ctor.
	(output_factory::output_factory): Pass dc to text_scheme_handler.
	(output_factory::get_scheme_handler): Make return non-const.
	(output_factory::make_sink): Move key-handling here, rather than
	in individual sinks.
	(context::handle_kv): New.
	(text_scheme_handler::make_sink): Eliminate key decoding.
	(text_scheme_handler::decode_kv): Convert to...
	(text_scheme_handler::maybe_handle_kv): ...this...
	(text_scheme_handler::get_keys): ...and this.
	(sarif_scheme_handler::make_sink): Eliminate key decoding.
	(sarif_scheme_handler::decode_kv): Convert to...
	(sarif_scheme_handler::maybe_handle_kv): ...this...
	(sarif_scheme_handler::get_keys): ...and this.
	(html_scheme_handler::make_sink): Eliminate key decoding.
	(html_scheme_handler::decode_kv): Convert to...
	(html_scheme_handler::maybe_handle_kv): ...this...
	(html_scheme_handler::get_keys): ...and this.
	(struct selftest::parser_test): Add "client_keys" arg, and update
	for new param ordering.
	(selftest::parser_test::parse_and_make_sink): New.
	(selftest::test_output_arg_parsing): Move auto-fixes to caller.
	(class selftest::test_key_handler): New.
	(selftest::test_client_arg_parsing): New test.
	(selftest::output_spec_cc_tests): Call it.
	* diagnostics/output-spec.h (class key_handler): New.
	(class scheme_handler): Move here from output-spec.cc.
	(context::report_unknown_key): Simplify params.
	(context::handle_kv): Update params.
	(context::context): Add "client_keys" param.
	(context::m_client_keys): New field.
	(struct dc_spec_context): Update order of params.  Add
	"client_keys" param.
	* libgdiagnostics.cc (spec_context::spec_context): Pass nullptr
	for client keys.
	* opts-diagnostic.cc (opt_spec_context::opt_spec_context):
	Likewise.  Update for new param ordering.

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