[clangd] Count number of references while merging RefSlabs inside FileIndex

Summary:
For counting number of references clangd was relying on merging every
duplication of a symbol. Unfortunately this does not apply to FileIndex(and one
of its users' BackgroundIndex), since we get rid of duplication by simply
dropping symbols coming from non-canonical locations. So only one or two(coming
from canonical declaration header and defined source file, if exists)
replications of the same symbol reaches merging step.

This patch changes reference counting logic to rather count number of different
RefSlabs a given SymbolID exists.

Reviewers: ilya-biryukov

Subscribers: ioeric, MaskRay, jkorous, mgrang, arphaman, jdoerfert, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D59481

llvm-svn: 360344