[clangd] Fix async index loading (from r341376).

Summary:
This wasn't actually async (due to std::future destructor blocking).
If it were, we would have clean shutdown issues if main returned
and destroyed Placeholder before the thread is done with it.

We could attempt to avoid any blocking by using shared_ptr or weak_ptr tricks so
the thread can detect Placeholder's destruction, but there are other potential
issues (e.g. loadIndex does tracing, and we'll destroy the tracer...)
Instead, once LSPServer::run returns, we wait for the index to finish loading
before exiting. Performance is not critical in this situation.

Reviewers: ilya-biryukov

Subscribers: ioeric, MaskRay, jkorous, arphaman, kadircet, cfe-commits

Differential Revision: https://reviews.llvm.org/D51674

llvm-svn: 341797