Fix small issues with -fgnat-encodings=minimal

Fix small issues with -fgnat-encodings=minimal

This is the mode where the GNAT compiler does not use special encodings
in the debug info to describe some Ada constructs, for example packed
array types.

	* gcc-interface/ada-tree.h (TYPE_PACKED_ARRAY_TYPE_P): Rename into...
	(TYPE_BIT_PACKED_ARRAY_TYPE_P): ...this.
	(TYPE_IS_PACKED_ARRAY_TYPE_P): Rename into...
	(BIT_PACKED_ARRAY_TYPE_P): ...this.
	(TYPE_IMPL_PACKED_ARRAY_P): Adjust to above renaming.
	* gcc-interface/gigi.h (maybe_pad_type): Remove IS_USER_TYPE..
	* gcc-interface/decl.c (gnat_to_gnu_entity) <E_Variable>: Adjust
	call to maybe_pad_type.
	<E_Ordinary_Fixed_Point_Type>: Remove const qualifiers for tree.
	<E_Signed_Integer_Subtype>: Remove redundant test and redundant call
	to associate_original_type_to_packed_array.  Turn into assertion.
	Call associate_original_type_to_packed_array and modify
	gnu_entity_name accordingly.  Explicitly set the parallel type
	for GNAT encodings.
	Call create_type_decl in the misaligned case before maybe_pad_type.
	<E_Array_Type>: Do not use the name of the implementation type for
	a packed array when not using GNAT encodings.
	<E_Array_Subtype>: Move around setting flags. Use the result of the
	call to associate_original_type_to_packed_array for gnu_entity_name.
	<E_Record_Subtype>: Create XVS type and XVZ variable only if debug
	info is requested for the type.
	Call create_type_decl if a padded type was created for a type entity
	(gnat_to_gnu_component_type): Use local variable and adjust calls to
	maybe_pad_type.
	(gnat_to_gnu_subprog_type): Adjust call to maybe_pad_type.
	(gnat_to_gnu_field): Likewise.
	(validate_size): Adjust to renaming of macro.
	(set_rm_size): Likewise.
	(associate_original_type_to_packed_array): Adjust return type and
	return the name of the original type if GNAT encodings are not used
	* gcc-interface/misc.c (gnat_get_debug_typ): Remove obsolete stuff.
	(gnat_get_fixed_point_type_info): Remove const qualifiers for tree.
	(gnat_get_array_descr_info): Likewise and set variables lazily.
	Remove call to maybe_debug_type.  Simplify a few computations.
	(enumerate_modes): Remove const qualifier for tree.
	* gcc-interface/utils.c (make_type_from_size): Adjust to renaming.
	(maybe_pad_type): Remove IS_USER_TYPE parameter and adjust.  Remove
	specific code for implementation types for packed arrays.
	(compute_deferred_decl_context): Remove const qualifier for tree.
	(convert): Adjust call to maybe_pad_type.
	(unchecked_convert): Likewise.
	* gcc-interface/utils2.c (is_simple_additive_expressio): Likewise.