[Ada] Add Max_Integer_Size attribute and couple of helper functions

[Ada] Add Max_Integer_Size attribute and couple of helper functions

gcc/ada/

	* doc/gnat_rm/implementation_defined_attributes.rst: Document the
	new Max_Integer_Size attribute.
	* gnat_rm.texi: Regenerate.
	* exp_attr.adb (Get_Integer_Type): Call Small_Integer_Type_For.
	(Expand_N_Attribute_Reference) <Attribute_Pred>: Replace selection
	code with call to Integer_Type_For.
	<Attribute_Succ>: Likewise.
	<Attribute_Val>: Likewise.
	<Attribute_Valid>: Likewise.
	<Attribute_Max_Integer_Size>: Raise Program_Error.
	* exp_ch3.adb (Expand_Freeze_Enumeration_Type): Replace selection
	code with call to Integer_Type_For.
	(Simple_Init_Initialize_Scalars_Type): Use Long_Long_Unsigned and
	System_Max_Integer_Size to size the largest integer type.
	* exp_pakd.adb (Compute_Linear_Subscript): Minor tweak.
	(Create_Packed_Array_Impl_Type): Use Small_Integer_Type_For.
	* exp_util.ads (Integer_Type_For): New function.
	(Small_Integer_Type_For): Likewise.
	* exp_util.adb (Adjust_Condition): Use Integer_Type_For.
	(Component_May_Be_Bit_Aligned): Use System_Max_Integer_Size.
	(Integer_Type_For): New function.
	(Small_Integer_Type_For): Likewise.
	(Matching_Standard_Type): Use Small_Integer_Type_For.
	(Needs_Constant_Address): Replace 64 with System_Max_Integer_Size.
	* freeze.adb (Set_Small_Size): Likewise.
	(Size_Known): Likewise.
	(Check_Suspicious_Modulus): Likewise.
	(Check_Large_Modular_Array): Likewise.
	(Freeze_Entity): Likewise.
	* layout.adb (Layout_Type): Likewise.
	* sem_aggr.adb: Add with and use clauses for Ttypes.
	(Resolve_Aggregate): Replace 64 with System_Max_Integer_Size.
	* sem_attr.ads (Attribute_Impl_Def): Add Attribute_Max_Integer_Size.
	* sem_attr.adb (Analyze_Attribute) <Attribute_Max_Integer_Size>: New
	(Eval_Attribute): Likewise.
	* sem_ch13.adb (Adjust_Record_For_Reverse_Bit_Order): Change max
	scalar size to System_Max_Integer_Size.
	(Check_Array_Type): Replace 64 with System_Max_Integer_Size and
	remove superfluous test.
	(OK_Component): Likewise.
	* sem_ch5.adb: Add with and use clauses for Ttypes.
	(Analyze_Assignment): Replace 64 with System_Max_Integer_Size.
	* snames.ads-tmpl (Name_Max_Integer_Size): New attribute name.
	(Attribute_Id): Add Attribute_Max_Integer_Size.
	* ttypes.ads (System_Max_Integer_Size): New constant.