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

[Pierre-Marie de Rodat]

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.

Tested on x86_64-pc-linux-gnu, committed on trunk

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.
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