[Ada] Improve error recovery for bad comma/semicolon in expression

[Arnaud Charlet <charlet at adacore dot com>]

This patch improves the error recovery for an errant comma or semicolon
after one condition in an expression when more conditions follow, as
shown in this example:

     1. procedure BadANDTHEN (X : Integer) is
     2. begin
     3.    if X > 10
     4.      and then X mod 4 = 2;
                                 |
        >>> extra ";" ignored

     5.      and then X mod 12 = 8
     6.    then
     7.       null;
     8.    end if;
     9. end;

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

2014-10-20  Robert Dewar  <dewar@adacore.com>

	* par-ch4.adb (P_Expression): Handle extraneous comma/semicolon
	in middle of expression with logical operators.

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