ada: Support Put_Image for types in user-defined instances of predefined generics.

ada: Support Put_Image for types in user-defined instances of predefined generics.

Predefined units do not generally support the Put_Image attribute.
There are good reasons for this in most cases. But if a user-defined
instantiation of a predefined generic occurs in Ada 2022 code, then
Put_Image can be supported for types declared therein. Add this support.

gcc/ada/

	* exp_put_image.adb (Put_Image_Enabled): Return True in more
	cases. In particular, when testing to see if a type occurs in a
	predefined unit, test the type's code unit
	(obtained by calling Get_Code_Unit). In the case of type within a
	user-defined instance of a predefined generic, Is_Predefined_Unit
	will return True for the type and False for the type's code unit.