ada: Fix too small secondary stack allocation for returned conversion

ada: Fix too small secondary stack allocation for returned conversion

The previous fix did not address a latent issue whereby the allocation
would be made using the (static) subtype of the conversion instead of
the (dynamic) subtype of the return object, so this change rewrites the
code responsible for determining the type used for the allocation, and
also contains a small improvement to the Has_Tag_Of_Type predicate.

gcc/ada/

	* exp_ch3.adb (Make_Allocator_For_Return): Rewrite the logic that
	determines the type used for the allocation and add assertions.
	* exp_util.adb (Has_Tag_Of_Type): Also return true for extension
	aggregates.