Fix Clang crash with template type diffing.

Fixes https://llvm.org/bugs/show_bug.cgi?id=27129 which is crash involving type
aliases and template type diffing.  Template arguments for type aliases and
template arguments for the underlying desugared type may not have one-to-one
relations, which could mess us the attempt to get more information from the
desugared type.  For type aliases, ignore the iterator over the desugared type.

llvm-svn: 264940