[analyzer] Fix liveness calculation for C++17 structured bindings

C++ structured bindings for non-tuple-types are defined in a peculiar
way, where the resulting declaration is not a VarDecl, but a
BindingDecl.
That means a lot of existing machinery stops working.

rdar://36912381

Differential Revision: https://reviews.llvm.org/D44956

llvm-svn: 328910