Fix codegen for structured binding binding in conditions

Summary:
The codegen for conditions assumes that a normal variable declaration is used in a condition, but this is not the case when a structured binding is used.

This fixes [PR36747](http://llvm.org/pr36747).

Thanks Nicolas Lesser for contributing the patch.

Reviewers: lichray, rsmith

Reviewed By: lichray

Subscribers: cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D44534

llvm-svn: 327780