[clangd] Code action for creating an ObjC initializer

The code action creates an initializer for the selected
ivars/properties, defaulting to all if only the interface/implementation
container is selected.

We add it based on the position of the first non initializer that we
see, and default to adding it where the @end token is.

We also use the ObjC parameter form of (nullable id) instead of
(id _Nullable) if the property has the nullable attribute.

Differential Revision: https://reviews.llvm.org/D116385