Move DraftMgr from ClangdServer to ClangdLSPServer

Summary:
This patch moves the draft manager closer to the edge of Clangd, from
ClangdServer to ClangdLSPServer.  This will make it easier to implement
incremental document sync, by making ClangdServer only deal with
complete documents.

As a result, DraftStore doesn't have to deal with versioning, and thus
its API can be simplified.  It is replaced by a StringMap in
ClangdServer holding a current version number for each file.

Signed-off-by: Simon Marchi <simon.marchi@ericsson.com>

Subscribers: klimek, mgorny, ilya-biryukov, jkorous-apple, ioeric, cfe-commits

Differential Revision: https://reviews.llvm.org/D44408

llvm-svn: 327711