[clangd] Introduce PostingList interface

This patch abstracts `PostingList` interface and reuses existing
implementation. It will be used later to test different `PostingList`
representations.

No functionality change is introduced, this patch is mostly refactoring
so that the following patches could focus on functionality while not
being too hard to review.

Reviewed By: sammccall, ioeric

Differential Revision: https://reviews.llvm.org/D51982

llvm-svn: 342155