ada: Move standard subtype declarations generation

ada: Move standard subtype declarations generation

Before this patch, the subtype declarations for Standard.Natural and
Standard.Positive were created before the entity for Standard.Integer
was complete. In preparation of a future change that will make it
impossible to call Etype on an incomplete node, this patch delays the
creation of these subtype declarations. It doesn't affect the behavior
of the compiler.

gcc/ada/ChangeLog:

	* cstand.adb (Create_Standard): Delay declaration generation for
	Natural and Positive.