[Ada] Remove out-of-range warning in unreachable code

[Ada] Remove out-of-range warning in unreachable code

This patch removes a warning in examples like this:

    if cond then
       return; -- or other jump
    end if;
    X := ...; -- where the value is out of range

where cond is known at compile time. It could, for example, be a generic
formal parameter that is known to be True in some instances.

As a side effect, this patch adds new warnings about unreachable code.

gcc/ada/

	* gnatls.adb (Output_License_Information): Remove pragma
	No_Return; call sites deal with Exit_Program.
	* libgnat/g-socthi.adb (C_Connect): Suppress warning about
	unreachable code.
	* sem_ch5.adb (Check_Unreachable_Code): Special-case if
	statements with static conditions.  If we remove unreachable
	code (including the return statement) from a function, add
	"raise Program_Error", so we won't warn about missing returns.
	Remove Original_Node in test for N_Raise_Statement; it's not
	needed.  Remove test for CodePeer_Mode; if Operating_Mode =
	Generate_Code, then CodePeer_Mode can't be True.  Misc cleanup.
	Do not reuse Nxt variable for unrelated purpose (the usage in
	the Kill_Dead_Code loop is entirely local to the loop).
	* sem_ch6.adb: Add check for Is_Transfer. Misc cleanup.
	* sem_prag.adb: Minor.
	* sem_res.adb: Minor.
	* sem_util.adb: Minor cleanup.
	(Is_Trivial_Boolean): Move to nonnested place, so it can be
	called from elsewhere.
	(Is_Static_Constant_Boolean): New function.
	* sem_util.ads (Is_Trivial_Boolean): Export.
	(Is_Static_Constant_Boolean): New function.