Fix bug in DynTypedMatcher::constructVariadic() that would cause false negatives.

Summary:
Change r219118 fixed the bug for anyOf and eachOf, but it is still
present for unless.
The variadic wrapper doesn't have enough information to know how to
restrict the type. Different operators handle restrict failures in
different ways.

Reviewers: klimek

Subscribers: klimek, cfe-commits

Differential Revision: http://reviews.llvm.org/D5731