This patch improves the synactic error recovery in the case where a ";"
is used instead of "," in a case expression. The following test should
get these errors:

case_exp_semi.ads:5:28: ";" should be ","

package Case_Exp_Semi is

   X : Integer := 1;
   Y : Integer :=
     (case X is when 1 => 1; when 2 => 2, when others => 3);

end Case_Exp_Semi;

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

2017-04-25  Bob Duff  <duff@adacore.com>

	* par-ch4.adb (P_Case_Expression): If a semicolon
	is followed by "when", assume that ";" was meant to be ",".