diagnostics: simplifying output-spec.cc

diagnostics: simplifying output-spec.cc

No functional change intended.

gcc/ChangeLog:
	* diagnostics/output-spec.cc: Rename "parsed_arg" to
	"scheme_and_kvs" throughout.  Rename "unparsed_arg" to
	"unparsed_spec" throughout, and make a member of
	output_spec::context rather than passing it around.
	* diagnostics/output-spec.h: Likewise.
	* libgdiagnostics.cc: Likewise.
	* opts-diagnostic.cc: Likewise.

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