Restored r303067 and fixed failing test.

Summary:
This commit restores r303067(reverted by r303094) and fixes the 'formatting.test'
failure.
The failure is due to destructors of `ClangdLSPServer`'s fields(`FixItsMap` and
`FixItsMutex`) being called before destructor of `Server`. It led to the worker
thread calling `consumeDiagnostics` after `FixItsMutex` and `FixItsMap`
destructors were called.
Also, clangd is now run with '-run-synchronously' flag in 'formatting.test'.

Reviewers: bkramer, krasimir

Reviewed By: krasimir

Subscribers: mgorny, cfe-commits, klimek

Differential Revision: https://reviews.llvm.org/D33233

llvm-svn: 303151