Update `DynTypedNode` to support the conversion of `TypeLoc`s.

This provides better support for `TypeLoc`s to allow `TypeLoc`-related
matchers to feature stricter typing and to avoid relying on the dynamic
casting of `TypeLoc`s in matchers.

Reviewed By: ymandel, tdl-g, sbenza

Differential Revision: https://reviews.llvm.org/D110586