[clangd] Add symbol slab size to index memory consumption estimates

Currently, `SymbolIndex::estimateMemoryUsage()` returns the "overhead"
estimate, i.e. the estimate of the Index data structure excluding
backing data (such as Symbol Slab and Reference Slab). This patch
propagates information about paired data size where necessary.

Reviewed By: ioeric, sammccall

Differential Revision: https://reviews.llvm.org/D51539

llvm-svn: 341800