Allow for redundant type checks for pattern matching

[mario-bucev]

Allows for ascribing type parameters to e.g. tuples patterns, such as in the following snippet:

def test[A, B, C](a: A, b: B, c: C): Unit = {
  // annotating aa, bb and cc with their respecting type
  val (aa: A, bb: B, cc: C) = (a, b, c)
}

This was previously rejected.
Note that the Scala 2 compiler will issue spurious warnings saying that these type tests cannot be checked at runtime (which is true, but they can be checked at compile time 😋).