[clangd] Improve document symbols support for Objective-C categories and methods

- Categories will now show up as `MyClass(Category)` instead of
  `Category` and `MyCategory()` instead of `(anonymous)` in document
  symbols

- Methods will now be shown as `-selector:` or `+selector:`
  instead of `selector:` to differentiate between instance and class
  methods in document symbols

Differential Revision: https://reviews.llvm.org/D96612