Ada: Fix internal error on access attribute used as subpool in allocator

Ada: Fix internal error on access attribute used as subpool in allocator

This is a regression present for quite a long time: the compiler aborts
on an allocator whose subpool name is an access attribute and when an
allocation procedure must be generated, for example when the allocation
is controlled.

The fix is to do what is done elsewhere in Build_Allocate_Deallocate_Proc,
that is to say pass the allocation procedure as the new scope in the call
to the New_Copy_Tree function.

gcc/ada/
	PR ada/124054
	* exp_util.adb (Build_Allocate_Deallocate_Proc): Tidy up and pass
	Proc_Id as the new scope in the call to the New_Copy_Tree function.

gcc/testsuite/
	* gnat.dg/allocator4.adb: New test.

Co-authored-by: Liam Powell <liam@liampwll.com>