[clangd] New conventions for JSON-marshalling functions, centralize machinery

Summary:
 - JSON<->Obj interface is now ADL functions, so they play nicely with enums
 - recursive vector/map parsing and ObjectMapper moved to JSONExpr and tested
 - renamed (un)parse to (de)serialize, since text -> JSON is called parse
 - Protocol.cpp gets a bit shorter

Sorry for the giant patch, it's prety mechanical though

Reviewers: ilya-biryukov

Subscribers: klimek, cfe-commits

Differential Revision: https://reviews.llvm.org/D40596

llvm-svn: 319478