Fix secondary constructor capture checking

[odersky]

Secondary constructors lost parameter captures. This is fixed now. As a result, scala.util.Random is now safe to use from safe mode.

Solution sketch: We keep track of which parameters in secondary constructors flow into a parameter of the primary constructor. For arguments of secondary constructors corresponding to such parameters we can use their actual capture sets. For primary constructor parameters that are not instantiated with an argument of a secondary constructors we assume that they keep their declared formal parameter capture set, which is a conservative appoximation.

[odersky]

@natsukagami ping for review

[natsukagami]

LGTM, just one note

[natsukagami]

This doesn't seem to work if the parameter has multiple alias annotations, like in this case:

import language.experimental.captureChecking

class T

class A(val x: T^, val y: T^):
  def this(v: T^) = this(v, v)

def f(x: T^) =
  val a = A(x)
  val b: T^{x} = a.x
  val c: T^{x} = a.y

This gives the following -Vprint:cc output:

package <empty> {
  import language.experimental.captureChecking
  @SourceFile("a.scala") class T() extends Object() {}
  @SourceFile("a.scala") class A(x: T^, y: T^) extends Object() {
    val x: T^
    val y: T^
    def <init>(@paramAlias("y") @paramAlias("x") v: T^): Unit =
      {
        this(v, v)
        ()
      }
  }
  final lazy module val a$package: a$package^{} = new a$package()
  @SourceFile("a.scala") final module class a$package() extends Object() {
    private[this] type $this = a$package.type
    private def writeReplace(): AnyRef =
      new scala.runtime.ModuleSerializationProxy(classOf[a$package.type])
    def f(x: T^): Unit =
      {
        val a: A{val x: T^; val y: T^{x}}^{any, x} = new A(x)
        val b: T^{x} = a.x
        val c: T^{x} = a.y
        ()
      }
  }
}

(and then b gets rejected)