Fix internal error on broken import of vector intrinsics

The change also makes small adjustments to warning messages for intrinsics.

gcc/ada/
	* gcc-interface/decl.c (gnat_to_gnu_subprog_type): Turn variable
	into constant.  Capitalize GCC in warning message.
	(intrin_arglists_compatible_p): Change parameter to pointer-to-const
	Adjust warning messages.  Turn warning into error for vector types.
	(intrin_return_compatible_p): Likewise.
	(intrin_profiles_compatible_p): Change parameter to pointer-to-const