[clangd] Run formatting operations asynchronously.

Summary:
They don't need ASTs or anything, so they should still run immediately.

These were sync for historical reasons (they predate clangd having a pervasive
threading model). This worked ok as they were "cheap".
Aside for consistency, there are a couple of reasons to make them async:
 - they do IO (finding .clang-format) so aren't trivially cheap
 - having TUScheduler involved in running these tasks means we can use it as
   an injection point for configuration.
   (TUScheduler::run will need to learn about which file is being operated on,
   but that's an easy change).
 - adding the config system adds more potential IO, too

Reviewers: kbobyrev

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D82642