Implement the basic approach for instantiating types, with a lot of FIXME'd

stubs for those types we don't yet know how to instantiate (everything
that isn't a template parameter!).

We now instantiate default arguments for template type parameters when
needed. This will be our testbed while I fill out the remaining
type-instantiation logic.



git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@65649 91177308-0d34-0410-b5e6-96231b3b80d8