Implement correct capture check for lazy vals

[odersky]

Compiler version

Minimized example

def f(c: T^, d: T^) = 
  lazy val x: T^{c} = d; c
  val f = () => x

Output

Compiling this with -Xprint:cc gives:

package <empty> {
  @SourceFile("tests/new/test.scala") class T() extends Object() {}
  final lazy module val test$package: test$package = new test$package()
  @SourceFile("tests/new/test.scala") final module class test$package() extends
    Object() {
    private[this] type $this = test$package.type
    private def writeReplace(): AnyRef =
      new scala.runtime.ModuleSerializationProxy(classOf[test$package.type])
    def f(c: T^, d: T^): Unit =
      {
        lazy val x: T^{c} =
          {
            d
            c
          }
        val f: () ->{x} (ex$1: caps.Exists) -> T^{x} =
          {
            def $anonfun(): T^{x} = x
            closure($anonfun)
          }
        ()
      }
  }
}

Expectation

f should also capture d, but this is not recorded.

Generally, we have spent no thought so far how to capture check lazy vals, and it looks interesting. Maybe someone wants to pick that up?