Evaluate lazy signatures hidden by prepended methods

[KaanOzkan]

Motivation

Fixes #2710

When prepend places a wrapper before a method whose sig remains
pending, looking up the wrapper returns no signature.

module Wrapper
  def call(...) = super
end

class Foo
  extend T::Sig

  sig { params(value: String).returns(String) }
  def call(value) = value

  prepend Wrapper
end

method = Foo.instance_method(:call)

# Before: signature_of(method) # => nil
# After:  signature_of(method) # => Foo#call signature

Implementation

Query the underlying method to evaluate the pending sig using T::Utils.signature_forMethod, then query the wrapper again after sorbet-runtime associates the signature with it.

Tests

On top of automated tests, tested on Core

[vinistock]

Out of curiosity, what happens if the prepended method doesn't match the arity of the super method?

module Wrapper
  def foo(a, b, c)
    super(a + b, c)
  end
end

class Bar
  prepend Wrapper

  #: (Integer, Integer) -> Integer
  def foo(a, b)
    a + b
  end
end