ada: Emit warnings for (some) ineffective static predicate tests

Generate a warning if a static predicate tests for a value that
does not belong to the parent subtype. For example, in
  subtype S is Positive with Static_Predicate => S not in 0 | 11 | 222;
the 0 is ineffective because Positive already excludes that value.
Generation of this new warning is controlled by the -gnatw_s switch,
which can also be enabled via -gnatwa.

gcc/ada/

	* warnsw.ads: Add a new element,
	Warn_On_Ineffective_Predicate_Test, to the Opt_Warnings_Enum
	enumeration type.
	* warnsw.adb: Bind "-gnatw_s" to the new
	Warn_On_Ineffective_Predicate_Test switch. Add the new switch to
	the set of switches enabled by -gnata .
	* sem_ch13.adb
	(Build_Discrete_Static_Predicate): Declare new local procedure,
	Warn_If_Test_Ineffective, which conditionally generates new
	warning. Call this new procedure when building a new element of an
	RList.
	* doc/gnat_ugn/building_executable_programs_with_gnat.rst:
	Document the -gnatw_s switch (and the corresponding -gnatw_S
	switch).
	* gnat_ugn.texi: Regenerate.

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