[clangd] Use Builder for symbol slabs, and use sorted-vector for storage

Summary:
This improves a few things:
 - the insert -> freeze -> read sequence is now enforced/communicated by the
   type system
 - SymbolSlab::const_iterator iterates over symbols, not over id-symbol pairs
 - we avoid permanently storing a second copy of the IDs, and the
   string map's hashtable

The slab size is now down to 21.8MB for the LLVM project.
Of this only 2.7MB is strings, the rest is #symbols * `sizeof(Symbol)`.
`sizeof(Symbol)` is currently 96, which seems too big - I think
SymbolInfo isn't efficiently packed. That's a topic for another patch!

Also added simple API to see the memory usage/#symbols of a slab, since
it seems likely we will continue to care about this.

Reviewers: ilya-biryukov

Subscribers: klimek, mgrang, cfe-commits

Differential Revision: https://reviews.llvm.org/D41506

llvm-svn: 321412