[clangd] SymbolOccurrences -> Refs and cleanup

Summary:
A few things that I noticed while merging the SwapIndex patch:
 - SymbolOccurrences and particularly SymbolOccurrenceSlab are unwieldy names,
   and these names appear *a lot*. Ref, RefSlab, etc seem clear enough
   and read/format much better.
 - The asymmetry between SymbolSlab and RefSlab (build() vs freeze()) is
   confusing and irritating, and doesn't even save much code.
   Avoiding RefSlab::Builder was my idea, but it was a bad one; add it.
 - DenseMap<SymbolID, ArrayRef<Ref>> seems like a reasonable compromise for
   constructing MemIndex - and means many less wasted allocations than the
   current DenseMap<SymbolID, vector<Ref*>> for FileIndex, and none for
   slabs.
 - RefSlab::find() is not actually used for anything, so we can throw
   away the DenseMap and keep the representation much more compact.
 - A few naming/consistency fixes: e.g. Slabs,Refs -> Symbols,Refs.

Reviewers: ioeric

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

Differential Revision: https://reviews.llvm.org/D51605

llvm-svn: 341368