Introduce an AttributedType, but don't actually use it anywhere yet.

Introduce an AttributedType, but don't actually use it anywhere yet.
The initial TreeTransform is a cop-out, but it's more-or-less equivalent
to what we were doing before, or rather what we're doing now and might
eventually stop doing in favor of using this type.
I am simultaneously intrigued by the possibilities of rebuilding a
dependent Attri

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@122942 91177308-0d34-0410-b5e6-96231b3b80d8