[clangd] Add textDocument/ast extension method to dump the AST

This is a mass-market version of the "dump AST" tweak we have behind
-hidden-features.
I think in this friendlier form it'll be useful for people outside clang
developers, which would justify making it a real feature.
It could be useful as a step towards lightweight clang-AST tooling in clangd
itself (like matcher-based search).

Advantages over the tweak:
 - simplified information makes it more accessible, likely somewhat useful
   without learning too much clang internals
 - can be shown in a tree view
 - structured information gives some options for presentation (e.g.
   icon + two text colors + tooltip in vscode)
 - clickable nodes jump to the corresponding code
Disadvantages:
 - a bunch of code to handle different node types
 - likely missing some important info vs dump-ast due to brevity/oversight
 - may end up chasing/maintaining support for the long tail of nodes

Demo with VSCode support: https://imgur.com/a/6gKfyIV

Differential Revision: https://reviews.llvm.org/D89571