[clangd][ObjC] Improve completions for protocols + category names

- Render protocols as interfaces to differentiate them from classes
  since a protocol and class can have the same name. Take this one step
  further though, and only recommend protocols in ObjC protocol completions.

- Properly call `includeSymbolFromIndex` even with a cached
  speculative fuzzy find request

- Don't use the index to provide completions for category names,
  symbols there don't make sense

Differential Revision: https://reviews.llvm.org/D132962