[clangd] Introduce PostingList interface
This patch abstracts PostingList interface and reuses existing
implementation. It will be used later to test different PostingList
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