ada: Fix spurious warning on unreferenced refinement constituents

ada: Fix spurious warning on unreferenced refinement constituents

Listing an object as a state refinement constituent shouldn't be
considered to be a reference, at least from the point of view of the
machinery for detecting objects that are never referenced or written
without being referenced.

This patch fixes a spurious warning that rarely occurred in practice but
was annoyingly emitted for minimal reproducers for issues related to
state abstractions.

Note: there are other pragmas that should be similarly recognized (e.g.
Depends, Global and their refined variants), but recognizing them
efficiently probably requires a dedicated utility routine (i.e. to avoid
traversal of the parent chain for every kind of pragma).

gcc/ada/

	* sem_prag.adb
	(Sig_Pragma): Change flag for pragma Refined_State to mean "not
	significant"; this is primarily for documentation, because the
	exact value of the flag is not really taken into account for
	Refined_State.
	(Is_Non_Significant_Pragma_Reference): Add special handling for
	pragma Refined_State.