IR: make IrDeclarationWithVisibility.visibility mutable

It was mutable anyway in many subclasses. This is needed in order to
avoid depending on concrete IR implementations in the JVM IR backend
where it's necessary to be able to change visibility.

compiler/fir/fir2ir/src/org/jetbrains/kotlin/fir/lazy/Fir2IrLazySimpleFunction.kt

@@ -72,8 +72,11 @@ class Fir2IrLazySimpleFunction(
     override val name: Name
         get() = fir.name
 
-    override val visibility: Visibility
+    override var visibility: Visibility
         get() = fir.visibility
+        set(_) {
+            error("Mutating Fir2Ir lazy elements is not possible")
+        }
 
     override val modality: Modality
         get() = fir.modality!!