[clangd] ObjC fixes for semantic highlighting and xref highlights

- highlight references to protocols in class/protocol/extension decls
- support multi-token selector highlights in semantic + xref highlights
  (method calls and declarations only)
- In `@interface I(C)`, I now references the interface and C the category
- highlight uses of interfaces as types
- added semantic highlightings of protocol names (as "interface") and
  category names (as "namespace").
  These are both standard kinds, maybe "extension" will be standardized...
- highlight `auto` as "class" when it resolves to an ObjC pointer
- don't highlight `self` as a variable even though the AST models it as one

Not fixed: uses of protocols in type names (needs some refactoring of
unrelated code first)

Differential Revision: https://reviews.llvm.org/D97617