[clangd] Clear the semantic of RefSlab::size.

Summary:
The RefSlab::size can easily cause confusions, it returns the number of
different symbols, rahter than the number of all references.

- add numRefs() method and cache it, since calculating it everytime is nontrivial.
- clear misused places.

Reviewers: sammccall

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

Differential Revision: https://reviews.llvm.org/D53389

llvm-svn: 344745