[clangd] BackgroundIndex rebuilds symbol index periodically.

Summary:
Currently, background index rebuilds symbol index on every indexed file,
which can be inefficient. This patch makes it only rebuild symbol index periodically.
As the rebuild no longer happens too often, we could also build more efficient
dex index.

Reviewers: ilya-biryukov, kadircet

Reviewed By: kadircet

Subscribers: dblaikie, MaskRay, jkorous, arphaman, jfb, cfe-commits

Differential Revision: https://reviews.llvm.org/D55770

llvm-svn: 349496