[Ada] Ignore pragmas Compile_Time_Error/Warning in GNATprove mode

[Pierre-Marie de Rodat <derodat at adacore dot com>]

GNATprove does not have sometimes the precise information of the
compiler about size of types and objects, so that it cannot evaluate the
expressions in pragma Compile_Time_Error/Warning the same way the
compiler does.  Thus, these pragmas should be ignored in GNATprove mode,
as it can neither verify them nor assume them (if the expression cannot
be evaluated at compile time, then the semantics for GNAT is to ignore
them).

Tested on x86_64-pc-linux-gnu, committed on trunk

2018-10-09  Yannick Moy  <moy@adacore.com>

gcc/ada/

	* sem_prag.adb (Process_Compile_Time_Warning_Or_Error): Rewrite
	pragmas as null statements in GNATprove mode.

--- gcc/ada/sem_prag.adb
+++ gcc/ada/sem_prag.adb
@@ -7570,6 +7570,17 @@ package body Sem_Prag is
       --  Start of processing for Process_Compile_Time_Warning_Or_Error
 
       begin
+         --  In GNATprove mode, pragmas Compile_Time_Error and
+         --  Compile_Time_Warning are ignored, as the analyzer may not have the
+         --  same information as the compiler (in particular regarding size of
+         --  objects decided in gigi) so it makes no sense to issue an error or
+         --  warning in GNATprove.
+
+         if GNATprove_Mode then
+            Rewrite (N, Make_Null_Statement (Loc));
+            return;
+         end if;
+
          Check_Arg_Count (2);
          Check_No_Identifiers;
          Check_Arg_Is_OK_Static_Expression (Arg2, Standard_String);