[AST] Support ConceptReference in DynTypedNode, add dump().

The dump() is not actually included recursively in any other nodes' dump,
as this is too verbose (similar to NNS) but useful in its own right.

It's unfortunate to not have the actual tests yet, but the DynTypedNode
tests are matcher-based and adding matchers is a larger task than
DynTypedNode support (but can't be done first).

(I've got a clangd change stacked on this that uses DynTypedNode and
dump(), and both work. I'll send a change for matchers next).

Differential Revision: https://reviews.llvm.org/D159300