[clangd] Remove JSON library in favor of llvm/Support/JSON

Summary:
The library has graduated from clangd to llvm/Support.
This is a mechanical change to move to the new API and remove the old one.

Main API changes:
 - namespace clang::clangd::json --> llvm::json
 - json::Expr --> json::Value
 - Expr::asString() etc --> Value::getAsString() etc
 - unsigned longs need a cast (due to r336541 adding lossless integer support)

Reviewers: ilya-biryukov

Subscribers: mgorny, ioeric, MaskRay, jkorous, omtcyfz, cfe-commits

Differential Revision: https://reviews.llvm.org/D49077

llvm-svn: 336549