Allow conditions to be decomposed with structured bindings

Summary:
This feature was discussed but not yet proposed.  It allows a structured binding to appear as a //condition//

    if (auto [ok, val] = f(...))

So the user can save an extra //condition// if the statement can test the value to-be-decomposed instead.  Formally, it makes the value of the underlying object of the structured binding declaration also the value of a //condition// that is an initialized declaration.

Considering its logicality which is entirely evident from its trivial implementation, I think it might be acceptable to land it as an extension for now before I write the paper.

Reviewers: rsmith, faisalv, aaron.ballman

Reviewed By: rsmith

Subscribers: aaron.ballman, cfe-commits

Differential Revision: https://reviews.llvm.org/D39284

llvm-svn: 320011