[AST] Fixed extraneous warnings for binary conditional operator

Summary:
Binary conditional operator gave warnings where ternary operators
did not. They have been fixed to warn similarly to ternary operators.

Link: https://bugs.llvm.org/show_bug.cgi?id=42239

Reviewers: rsmith, aaron.ballman, nickdesaulniers

Reviewed By: rsmith, nickdesaulniers

Subscribers: srhines, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D63369

llvm-svn: 363857