[clangd] workspace/symbol should be async, it reads from the index.

Summary:
To enable this, TUScheduler has to provide a way to run async tasks without
needing a preamble or AST!

Reviewers: ilya-biryukov

Subscribers: javed.absar, ioeric, MaskRay, jkorous, arphaman, jfb, kadircet, cfe-commits

Differential Revision: https://reviews.llvm.org/D53644

llvm-svn: 345268