Make the InjectedClassNameType the canonical type of the current instantiation

Make the InjectedClassNameType the canonical type of the current instantiation
of a class template or class template partial specialization.  That is to
say, in
  template <class T> class A { ... };
or
  template <class T> class B<const T*> { ... };
make 'A<T>' and 'B<const T*>' sugar for the corresponding InjectedClassNameType
when written inside the appropriate context.  This allows us to track the
current instantiation appropriately even inside AST routines.  It also allows
us to compute a DeclContext for a type much more efficiently, at some extra
cost every time we write a template specialization (which can be optimized,
but I've left it simple in this patch).

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@102407 91177308-0d34-0410-b5e6-96231b3b80d8