[clangd] Implement findOccurrences interface in dynamic index.

Summary:
Implement the interface in
  - FileIndex
  - MemIndex
  - MergeIndex

Depends on https://reviews.llvm.org/D50385.

Reviewers: sammccall, ilya-biryukov

Reviewed By: sammccall

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

Differential Revision: https://reviews.llvm.org/D51279

llvm-svn: 341242