[clangd] Track document versions, include them with diags, enhance logs

Summary:
This ties to an LSP feature (diagnostic versioning) but really a lot
of the value is in being able to log what's happening with file versions
and queues more descriptively and clearly.

As such it's fairly invasive, for a logging patch :-\

Key decisions:
 - at the LSP layer, we don't reqire the client to provide versions (LSP
   makes it mandatory but we never enforced it). If not provided,
   versions start at 0 and increment. DraftStore handles this.
 - don't propagate magically using contexts, but rather manually:
   addDocument -> ParseInputs -> (ParsedAST, Preamble, various callbacks)
   Context-propagation would hide the versions from ClangdServer, which
   would make producing good log messages hard
 - within ClangdServer, treat versions as opaque and unordered.
   std::string is a convenient type for this, and allows richer versions
   for embedders. They're "mandatory" but "null" is a reasonable default.

Subscribers: ilya-biryukov, javed.absar, MaskRay, jkorous, arphaman, kadircet, usaxena95, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D75582