[clangd] Support ConceptReference in generic AST wrangling code

Now we can store it in DynTypedNode, we can target these nodes
(SelectionTree) and resolve them (FindTarget).
This makes Hover, go-to-def etc work in all(?) cases.

Also support it in DumpAST.

Differential Revision: https://reviews.llvm.org/D159299

GitOrigin-RevId: a8c9b9f1402ccf93955a26d5933cd8c3150c3845
6 files changed