Fix type name for prototype rbi

[ksss]

This PR solves two problems.

Fix untyped in case of COLON3

class Test
  sig { returns(Foo) }
  def m1; end

  sig { returns(::Foo) }
  def m2; end
end

class Test
  def m1: () -> Foo

  # before
  def m2: () -> untyped

  # after
  def m2: () -> ::Foo
end

Fixed a problem where namespace disappears when an absolute path is specified

class Test
  sig { returns(Foo::Bar) }
  def m3; end

  sig { returns(::Foo::Bar) }
  def m4; end
end

class Test
  def m3: () -> Foo::Bar

  # before
  def m4: () -> ::Bar

  # after
  def m4: () -> ::Foo::Bar
end

[soutaro]

👍