[Ada] Avoid unwanted warnings for statically-known-successful assertions

[Ada] Avoid unwanted warnings for statically-known-successful assertions

The -gnatwc switch enables warnings for test condition outcomes that are
known at compile time. Such warnings are unlikely to be useful in the
case of an assertion expression (or a subexpression thereof), so do not
generate them in that case.

gcc/ada/

	* sem_warn.adb (Warn_On_Constant_Valid_Condition): Do not
	generate a warning if the expression in question is an assertion
	expression, or a subexpression thereof. But do call
	Test_Comparison so that it can generate warnings for the cases
	that it generates warnings for.
	* sem_prag.ads: Modify Assertion_Expression_Pragma constant so
	that the predicate Sem_Util.In_Assertion_Expression_Pragma
	returns True for the expression of a Compile_Time_Error pragma.