[Ada] Avoid confusing warning on exception propagation in GNATprove mode

[Ada] Avoid confusing warning on exception propagation in GNATprove mode

When compiling with the restriction No_Exception_Propagation, GNAT compiler
may issue a warning about exceptions not being propagated. This warning is
useless and confusing to users for GNATprove analysis, as GNATprove
precisely detects possible exceptions, so disable the warning in that mode.

2018-07-17  Yannick Moy  <moy@adacore.com>

gcc/ada/

	* gnat1drv.adb (Gnat1drv): Do not issue warning about exception not
	being propagated in GNATprove mode.

From-SVN: r262781