[Ada] Revise Storage_Model_Support operations to do checks and take objects and types

[Ada] Revise Storage_Model_Support operations to do checks and take objects and types

The functions in subpackage Storage_Model_Support (apart from the
Has_*_Aspect functions) are revised to have assertions that will fail
when passed a parameter that doesn't specify the appropriate aspect
(either aspect Storage_Model_Type or Designated_Storage_Model), instead
of returning Empty for bad arguments. Also, various of the functions now
allow either a type with aspect Storage_Model_Type or an object of such
a type.

gcc/ada/

	* sem_util.ads (Storage_Model_Support): Revise comments on most
	operations within this nested package to reflect that they can
	now be passed either a type that has aspect Storage_Model_Type
	or an object of such a type.  Change the names of the relevant
	formals to SM_Obj_Or_Type. Also, add more precise semantic
	descriptions in some cases, and declare the subprograms in a
	more logical order.
	* sem_util.adb (Storage_Model_Support.Storage_Model_Object): Add
	an assertion that the type must specify aspect
	Designated_Storage_Model, rather than returning Empty when it
	doesn't specify that aspect.
	(Storage_Model_Support.Storage_Model_Type): Add an assertion
	that formal must be an object whose type specifies aspect
	Storage_Model_Type, rather than returning Empty for when it
	doesn't have such a type (and test Has_Storage_Model_Type_Aspect
	rather than Find_Value_Of_Aspect).
	(Storage_Model_Support.Get_Storage_Model_Type_Entity): Allow
	both objects and types, and add an assertion that the type (or
	the type of the object) has a value for aspect
	Storage_Model_Type.