Remove unnececary boxing in `a.synchronized(1)`

[DarkDimius]

Both Scalac and Dotty assume that synchronized takes and returns an Object after erasure.
Actually it doesn't so there's no need to box.
We could potentially leave synchronized as a polymorphic method after erasure and save a bit on GC pressure and bytecode size.

[smarter]

Fixed in scalac: scala/scala@92d1af1

[allanrenucci]

Raising the priority since it affects thread safe lazy vals. For example:

def test = {
  lazy val foo = 1
  ...
}

compiles to:

def test = {
  lazy var foo$lzy1: dotty.runtime.LazyInt = new dotty.runtime.LazyInt()
  def foo$lzyINIT1(): Int =
    Int.unbox(
      foo$lzy1.synchronized(
        Int.box(
          if (foo$lzy1.initialized()) foo$lzy1.value() else {
            foo$lzy1.value_=(1)
            foo$lzy1.initialized_=(true)
            foo$lzy1.value()
          }
        )
      )
    )
  def foo(): Int =
    if (foo$lzy1.initialized()) foo$lzy1.value() else foo$lzyINIT1()
  ...
}