Index private_class_method / public_class_method calls

[alexcrocha]

Part of #89

This PR adds indexer support for private_class_method and public_class_method calls. Each call becomes a SingletonMethodVisibilityDefinition that the resolver applies to the singleton method declaration in a follow-up PR. This is the base of a Graphite stack.

We need a definition because the indexer can't yet attach the visibility change to the singleton method:

class Foo
  def self.bar; end
  private_class_method :bar
end

The method declaration for Foo::<Foo>#bar() only exists after resolution.

Why a new definition type

I made it a separate type rather than adding a receiver field to MethodVisibilityDefinition. Foo.private_class_method :bar accepts an explicit receiver, but private / protected / public don't, so MethodVisibilityDefinition has no receiver field today. Adding receiver: Option<NameId> to the existing type would cost every call site. Constant visibility made the same split.

In this PR

• add Definition::SingletonMethodVisibility and SingletonMethodVisibilityDefinition
• expose the new kind through the Rust / C / Ruby definition APIs
• index private_class_method / public_class_method calls with literal symbol or string targets, with or without an explicit constant receiver
• diagnose top-level calls, dynamic receivers, and non-literal arguments

Not in this PR

A few things are intentionally deferred to follow-up PRs in the stack:

• extracting the receiver-validation and literal-extraction helpers we duplicate from handle_constant_visibility
• supporting array arguments (private_class_method [:a, :b])
• supporting inline-def arguments (private_class_method def self.foo; end)

[alexcrocha]

• Extract shared helpers for visibility-call processing #783
• Support array and inline-def arguments to private_class_method #782
• Resolve retroactive singleton method visibility changes #781
• Index private_class_method / public_class_method calls #780 👈 (View in Graphite)
• main

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