[Ada] fix wrong code for component of constrained subtype

[Olivier Hainque]

Olivier Hainque wrote:
> 
> The attached patch fixes a regression compared to the GCC 3.4 series.

 Forgot to say: bootstrapped and regtested for languages=c,ada on
 x86_64-pc-linux-gnu.

> 	* utils.c (type_for_nonaliased_component_p): Return false for
> 	all AGGREGATE_TYPEs.
> 
> 	* gnat.dg/nested_subtype_byref.ad[bs]: Part of new test.
> 	* gnat.dg/test_nested_subtype_byref.adb: New test.