Reduce usage of limited_with clauses with -fdump-ada-spec

Reduce usage of limited_with clauses with -fdump-ada-spec

The problem is that subtypes are not part of the limited view of a package
so we need to use types in conjunction with limited_with clauses, which is
not always desirable as this yields less portable Ada bindings.  The patch
also contains a small enhancement for complex floating-point types.

gcc/c-family/
	* c-ada-spec.cc (dump_ada_node) <COMPLEX_TYPE>: Deal with usual
	floating-point complex types.
	<POINTER_TYPE>: Do not use limited_with clause if the designated
	type is a scalar type.