[clangd] Add parsing and value inspection to JSONExpr.

Summary:
This will replace the places where we're using YAMLParser to parse JSON now:
  - the new marshalling code (T::parse()) should handle fewer cases and require
    fewer explicit casts
  - we'll early-reject invalid JSON that YAMLParser accepts
  - we'll be able to fix protocol-parsing bugs caused by the fact that YAML can
    only parse forward

I plan to do the conversion as soon as this lands, but I don't want it in one
patch as the protocol.cpp changes are conflict-prone.

Reviewers: ioeric

Subscribers: ilya-biryukov, cfe-commits

Differential Revision: https://reviews.llvm.org/D40182

llvm-svn: 318774