[JS IR DCE] Improve handling overrides

Introduce the notion of contagious for overridable declarations.
So, now overriding uncontagious declaration will not lead to becoming a declaration reachable.
By default, all declarations treated as contagious, it's not the most efficient, but it's safest.
In case when we access a declaration through a fake-override declaration,
the original (real) one will not be marked as contagious, so, later,
other overrides will not be processed unconditionally only because
it overrides a reachable declaration.

compiler/ir/backend.common/src/org/jetbrains/kotlin/backend/common/ir/IrUtils.kt

@@ -1,5 +1,5 @@
 /*
- * Copyright 2010-2019 JetBrains s.r.o. and Kotlin Programming Language contributors.
+ * Copyright 2010-2020 JetBrains s.r.o. and Kotlin Programming Language contributors.
  * Use of this source code is governed by the Apache 2.0 license that can be found in the license/LICENSE.txt file.
  */
 
@@ -106,6 +106,12 @@ val IrSimpleFunction.isOverridable: Boolean
 
 val IrSimpleFunction.isOverridableOrOverrides: Boolean get() = isOverridable || overriddenSymbols.isNotEmpty()
 
+val IrDeclaration.isMemberOfOpenClass: Boolean
+    get() {
+        val parentClass = this.parent as? IrClass ?: return false
+        return !parentClass.isFinalClass
+    }
+
 fun IrReturnTarget.returnType(context: CommonBackendContext) =
     when (this) {
         is IrConstructor -> context.irBuiltIns.unitType