Patter match waring on erased type not needed

[nicolasstucki]

class Foo {
  def foo[A](lss: List[List[A]]): Unit = {
    lss match {
      case xss: List[List[A]] =>
    }
  }
}

produces the following waring

-- [E091] Pattern Match Exhaustivity Warning: Foo.scala:5:16 -------------------
5 |      case xss: List[List[A]] =>
  |                ^^^^^^^^^^^^^
  |       type arguments are not checked since they are eliminated by erasure

even if we know statically that the type will match.

[allanrenucci]

Simpler test case:

class Foo {
  def foo(lss: List[Int]): Unit = {
    lss match {
      case xss: List[Int] =>
    }
  }
}

-- [E091] Pattern Match Exhaustivity Warning: tests/allan/Test.scala:4:16 ------
4 |      case xss: List[Int] =>
  |                ^^^^^^^^^
  |       type arguments are not checked since they are eliminated by erasure

longer explanation available when compiling with `-explain`

[allanrenucci]

I don't think we should emit unchecked warning in pattern matching. Otherwise will have to repeat the work for isInstanceOf checks: #3324.
I believe we can do it once for isInstanceOf checks after we transform pattern match cases to isInstanceOf checks.