[Ada] Fix incorrect itype sharing for case expression in limited type return

[Ada] Fix incorrect itype sharing for case expression in limited type return

The compiler aborts with an internal error in gigi, but the problem is an
itype incorrectly shared between several branches of an if_statement that
has been created for a Build-In-Place return.

Three branches of this if_statement contain an allocator statement and
the latter two have been obtained as the result of calling New_Copy_Tree
on the first; now the initialization expression of the first had also been
obtained as the result of calling New_Copy_Tree on the original tree, and
these chained calls to New_Copy_Tree run afoul of an issue with the copy
of itypes after the rewrite of an aggregate as an expression with actions.

Fixing this issue looks quite delicate, so this fixes the incorrect sharing
by replacing the chained calls to New_Copy_Tree with repeated calls on the
original expression, which is more elegant in any case.

gcc/ada/

	* exp_ch3.adb (Make_Allocator_For_BIP_Return): New local function.
	(Expand_N_Object_Declaration): Use it to build the three allocators
	for a Build-In-Place return with an unconstrained type.  Update the
	head comment after other recent changes.