ada: Fix small inaccuracy in previous change

The call to Build_Allocate_Deallocate_Proc must occur before the special
accessibility check for class-wide allocation is generated, because this
check comes with cleanup code.

gcc/ada/

	* exp_ch4.adb (Expand_Allocator_Expression): Move the first call to
	Build_Allocate_Deallocate_Proc up to before the accessibility check.