[Ada] Warn about unreachable code after calls with No_Return

[Ada] Warn about unreachable code after calls with No_Return

GNAT was already warning about unreachable code after raise/goto/exit
statements, but not after calls to procedures with No_Return. Now this
warning is extended.

Also, previously the warning was suppressed for unreachable RETURN after
RAISE statements. Now this suppression is narrowed to functions, because
only in function such a RETURN statement might be indeed needed (where
it is the only RETURN statement of a function).

gcc/ada/

	* sem_ch5.adb (Check_Unreachable_Code): Extend suppression to
	calls with No_Return aspect, but narrow it to functions.
	* sem_res.adb (Resolve_Call): Warn about unreachable code after
	calls with No_Return.