Fix a problem in ExpressionParser leading to trailing comments affecting indentation of an expression after a line break.

Summary:
E.g. the second line in

return aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa +
           b; //

is indented 4 characters more than in

return aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa +
       b;

Reviewers: klimek

Reviewed By: klimek

CC: cfe-commits

Differential Revision: http://llvm-reviews.chandlerc.com/D984