Initial stab at a generalized operation for determining the

Initial stab at a generalized operation for determining the
instantiation of a declaration from the template version (or version
that lives in a template) and a given set of template arguments. This
needs much, much more testing, but it suffices for simple examples
like

  typedef T* iterator;
  iterator begin();

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@72461 91177308-0d34-0410-b5e6-96231b3b80d8