ada: Improve detection of deactivated code for warnings with -gnatwt

ada: Improve detection of deactivated code for warnings with -gnatwt

Switch -gnatwt is used in GNAT to track deleted code. It can be emitted
by GNAT on code that is intentionally deactivated for a given configuration.
The current test to suppress spurious warnings is not complex enough to
detect all such cases. Now improved, by using the same test as used in
GNATprove to suppress warnings related to a "statically disabled condition
which evaluates to a given value", as described in SPARK UG 7.3.2.

gcc/ada/

	* exp_util.adb (Is_Statically_Disabled): New function to detect a
	"statically disabled condition which evaluates to a given value",
	as described in SPARK UG 7.3.2.
	(Kill_Dead_Code): Call the new function Is_Statically_Disabled for
	conditions of if statements.
	* exp_util.ads (Is_Statically_Disabled): New function spec.