Change description of fat pointertype with -fgnat-encodings=minimal

Change description of fat pointertype  with -fgnat-encodings=minimal

This makes a step back in the representation of fat pointer types in
the debug info with -fgnat-encodings=minimal so as to avoid hiding the
data indirection and making it easiser to synthetize the construct.

gcc/ada/ChangeLog
	* gcc-interface/decl.c (gnat_to_gnu_entity) <E_Array_Type>: Add a
	description of the various types associated with the unconstrained
	type.  Declare the fat pointer earlier.  Set the current function
	as context on the template type, and the fat pointer type on the
	array type.  Always mark the fat pointer type as artificial and set
	it as the context for the pointer type to the array.  Also reuse
	GNU_ENTITY_NAME.  Finish up the unconstrained type at the very end.
	* gcc-interface/misc.c (gnat_get_array_descr_info): Do not handle
	fat pointer types and tidy up accordingly.
	* gcc-interface/utils.c (build_unc_object_type): Do not set the
	context on the template type.