[clangd] Added move-only function helpers.

Summary:
They are now used in ClangdScheduler instead of deferred std::async
computations.
The results of `std::async` are much less effective and do not provide
a good abstraction for similar purposes, i.e. for storing additional callbacks
to clangd async tasks. The actual callback API will follow a bit later.

Reviewers: klimek, bkramer, sammccall, krasimir

Reviewed By: sammccall

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D38627

llvm-svn: 315210