[Ada] Do not give warnings for compiler-generated entities by default

The rationale is that these entities are almost always the result of
expansion activities in the front-end, over which the user has very
limited control.  These warnings can be restored by means of -gnatD.

gcc/ada/

	* gcc-interface/utils.cc (gnat_pushdecl): Also set TREE_NO_WARNING
	on the decl if Comes_From_Source is false for the associated node.