ada: Restore default size for dynamic allocations of discriminated type

The allocation strategy for objects of a discriminated type with defaulted
discriminants is not the same when the allocation is dynamic as when it is
static (i.e a declaration): in the former case, the compiler allocates the
default size whereas, in the latter case, it allocates the maximum size.

This restores the default size, which was dropped during the refactoring.

gcc/ada/

	* exp_aggr.adb (Build_Array_Aggr_Code): Pass N in the call to
	Build_Initialization_Call.
	(Build_Record_Aggr_Code): Likewise.
	(Convert_Aggr_In_Object_Decl): Likewise.
	(Initialize_Discriminants): Likewise.
	* exp_ch3.ads (Build_Initialization_Call): Replace Loc witn N.
	* exp_ch3.adb (Build_Array_Init_Proc): Pass N in the call to
	Build_Initialization_Call.
	(Build_Default_Initialization): Likewise.
	(Expand_N_Object_Declaration): Likewise.
	(Build_Initialization_Call): Replace Loc witn N parameter and add
	Loc local variable.  Build a default subtype for an allocator of
	a discriminated type with defaulted discriminants.
	(Build_Record_Init_Proc): Pass the declaration of components in the
	call to Build_Initialization_Call.
	* exp_ch6.adb (Make_CPP_Constructor_Call_In_Allocator): Pass the
	allocator in the call to Build_Initialization_Call.