[clangd] Factor out the data-swapping functionality from MemIndex/DexIndex.

Summary:
This is now handled by a wrapper class SwapIndex, so MemIndex/DexIndex can be
immutable and focus on their job.

Old and busted:
 I have a MemIndex, which holds a shared_ptr<vector<Symbol*>>, which keeps the
 symbol slab alive. I update by calling build(shared_ptr<vector<Symbol*>>).

New hotness: I have a SwapIndex, which holds a unique_ptr<SymbolIndex>, which
 holds a MemIndex, which holds a shared_ptr<void>, which keeps backing
 data alive.
 I update by building a new MemIndex and calling SwapIndex::reset().

Reviewers: kbobyrev, ioeric

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

Differential Revision: https://reviews.llvm.org/D51422

llvm-svn: 341318