[clangd] Merge binary + YAML serialization behind a (mostly) common interface.

Summary:
Interface is in one file, implementation in two as they have little in common.
A couple of ad-hoc YAML functions left exposed:
 - symbol -> YAML I expect to keep for tools like dexp
 - YAML -> symbol is used for the MR-style indexer, I think we can eliminate
   this (merge-on-the-fly, else use a different serialization)

Reviewers: kbobyrev

Subscribers: mgorny, ilya-biryukov, ioeric, MaskRay, jkorous, arphaman, kadircet, cfe-commits

Differential Revision: https://reviews.llvm.org/D52453

llvm-svn: 342999