Suppress spurious Suppression

[som-snytt]

Guard against multiple registrations of nowarn on related elements.

Fixes #18341

[som-snytt]

By making it a case class so toString is useful, I can tell that not all the suppressions were the same. Here it's visible that end is different in the second suppression, so discarding it is bad.

ADD Suppression(tests/warn/i18341.scala:<296..303>,List(Any),296,296,false)

ADD Suppression(tests/warn/i18341.scala:<296..303>,List(Any),296,446,false)

In fact the duplication is because it sticks to everything:

  class B(@nowarn() useless: Int) extends Object() {
    @nowarn() private[this] val useless: Int
  }
  class C(@nowarn("msg=unused") useless: Int) extends Object() {
    @nowarn("msg=unused") private[this] val useless: Int
  }
  class D(useless: Int) extends Object() {
    private[this] val useless: Int
  }
  class E(@nowarn() useful: Int) extends Object() {
    @nowarn() private[this] val useful: Int
    def e: Int = E.this.useful.*(10)
  }
  class X() extends Object() {
    def extensionInCompanion: String = ???
  }
  @nowarn() final lazy module val X: X = new X()
  @nowarn() final module class X() extends Object() { this: X.type =>
    implicit def companionConversion(x: X): B = ???
    extension (x: X) def extensionInCompanion: String = ???
  }

Does this make the annotation useless? Well, check unused does say that a param is used iff its param accessor is used. Both should warn or neither; similarly for modules in whatever you call their dual nature. Normally, the "duplicate" warning would be ignored at the same position, assuming the message text also aligns.

But that is nonsense. The sensible way is to ignore duplicate nowarns with equal (start, end) and also of zero extent.