SI-6943 warn on value class miscomparison.

There's a very dangerous situation running around when you
combine universal equality with value classes:

  // All over your code
  val x = "abc"
  if (x == "abc") ...

  // Hey let's make x a value class
  val x = new ValueClass("abc")
  // Uh-oh

There was until now no warning when comparing a value class
with something else. Now there is.
Merge pull request #2444 from paulp/pr/value-class-cmp-warning

SI-6943 warn on value class miscomparison.