[clangd] Move DraftStore from ClangdLSPServer into ClangdServer.

ClangdServer already gets notified of every change, so it makes sense for it to
be the source of truth.
This is a step towards having ClangdServer expose a FS that includes dirty
buffers: D94554

Related changes:
 - version is now optional for ClangdServer, to preserve our existing fuzziness
   in this area (missing version ==> autoincrement)
 - ClangdServer::format{File,Range} are now more regular ClangdServer functions
   that don't need the code passed in. While here, combine into one function.
 - incremental content update logic is moved from DraftStore to
   ClangdLSPServer, with most of the implementation in SourceCode.cpp.
   DraftStore is now fairly trivial, and will probably ultimately be
   *replaced* by the dirty FS stuff.

Differential Revision: https://reviews.llvm.org/D97738