Don't generate setters for trivial private property setters

This change will prevent the compiler for generating Java bytecode for
private property setters that are trivial.

Since Kotlin uses direct field access for private properties, it will result
in the private setter never been used and since it cannot be accessed by any
other class without reflection, the setter cannot be covered by code
coverage tools.

See https://youtrack.jetbrains.com/issue/KT-20344 for the related YouTrack
issue.

compiler/backend/src/org/jetbrains/kotlin/codegen/PropertyCodegen.java

@@ -210,6 +210,12 @@ public class PropertyCodegen {
             return !isDefaultAccessor;
         }
 
+        // Non-private properties with private setter should not be generated for trivial properties
+        // as the class will use direct field access instead
+        if (accessor != null && accessor.isSetter() && Visibilities.isPrivate(descriptor.getSetter().getVisibility())) {
+            return !isDefaultAccessor;
+        }
+
         return true;
     }