d: Add `@visibility' and `@hidden' attributes.

The `@visibility' attribute is functionality the same as
`__attribute__((visibility))', and `@hidden' is a convenience alias to
`@visibility("hidden")' defined in the `gcc.attributes' module.

As the visibility of a symbol is also indirectly controlled by the
`export' keyword, the handling of this in the code generation pass has
been improved so that conflicts will be appropriately diagnosed.

gcc/d/ChangeLog:

	* d-attribs.cc (d_langhook_attribute_table): Add visibility.
	(insert_type_attribute): Use decl_attributes instead of
	merge_attributes.
	(insert_decl_attribute): Likewise.
	(apply_user_attributes): Do nothing when no UDAs applied.
	(d_handle_visibility_attribute): New function.
	* d-gimplify.cc (d_gimplify_binary_expr): Adjust.
	* d-tree.h (set_visibility_for_decl): Declare.
	* decl.cc (get_symbol_decl): Move setting of visibility flags to...
	(set_visibility_for_decl): ... here.  New function.
	* types.cc (TypeVisitor::visit (TypeStruct *)): Call
	set_visibility_for_decl().
	(TypeVisitor::visit (TypeClass *)): Likewise.

gcc/testsuite/ChangeLog:

	* gdc.dg/attr_visibility1.d: New test.
	* gdc.dg/attr_visibility2.d: New test.
	* gdc.dg/attr_visibility3.d: New test.

libphobos/ChangeLog:

	* libdruntime/gcc/attributes.d (visibility): Define.
	(hidden): Define.