[Ada] Ignore pragmas Compile_Time_Error/Warning in GNATprove mode

[Ada] Ignore pragmas Compile_Time_Error/Warning in GNATprove mode

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).

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.

From-SVN: r264981