[C++17] Allow an empty expression in an if init statement

Summary:
This fixes [PR35381](https://llvm.org/pr35381) and an additional bug where clang didn't warn about the C++17 extension when having an expression in the init statement.

Thanks Nicolas Lesser for contributing the patch.

Reviewers: rsmith

Reviewed By: rsmith

Subscribers: erik.pilkington, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D40445

llvm-svn: 327782