Fix duplicate name issues in output of -fdump-ada-spec

Fix duplicate name issues in output of -fdump-ada-spec

The namespace rules are different in the C family of languages and
in Ada, and a few adjustments are further needed in -fdump-ada-spec
because of them.

gcc/c-family/
	* c-ada-spec.c (dump_ada_enum_type): Dump a prefix for constants.
	(htable_t): New typedef.
	(overloaded_names): Use it.
	(add_name): New function.
	(init_overloaded_names): Use add_name to populate the table and add
	special cases for sigaction and stat.
	(overloaded_name_p): Rename into...
	(overloading_index): ...this.  Do not initialize overloaded_names table
	here.  Return the index or zero.
	(dump_ada_declaration): Minor tweaks.  Do not skip overloaded functions
	but add an overloading suffix instead.
	(dump_ada_specs): Initialize overloaded_names tables here.