Fix crash on invalid in template type diffing.

This is a fix for PR15895, where Clang will crash when trying to print a
template diff and the template uses an address of operator.  This resulted
from expecting a DeclRefExpr when the Expr could have also been
UnaryOperator->DeclRefExpr.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@181365 91177308-0d34-0410-b5e6-96231b3b80d8