Don't make -gsplit-dwarf imply -g

-gsplit-dwarf introduces order dependency: it overrides previous -g0 and -g1.

Don't imply -g so that it can be plugged into a build without worrying
that unnecessary debugging information may be generated.

2020-05-13  Fangrui Song  <maskray@google.com>

	PR debug/95096
	* opts.c (common_handle_option): Don't make -gsplit-dwarf imply -g.
	* doc/invoke.texi (-gsplit-dwarf): Update documentation.