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/tests/org/jetbrains/kotlin/codegen/PropertyGenTest.java

@@ -124,9 +124,10 @@ public class PropertyGenTest extends CodegenTestCase {
         getFoo.setAccessible(true);
         assertTrue((getFoo.getModifiers() & Modifier.PROTECTED) != 0);
         assertEquals(349, getFoo.invoke(instance));
-        Method setFoo = findDeclaredMethodByName(aClass, "setFoo");
-        setFoo.setAccessible(true);
-        assertTrue((setFoo.getModifiers() & Modifier.PRIVATE) != 0);
+        // See KT-20344
+        // Method setFoo = findDeclaredMethodByName(aClass, "setFoo");
+        // setFoo.setAccessible(true);
+        // assertTrue((setFoo.getModifiers() & Modifier.PRIVATE) != 0);
         Method setter = findDeclaredMethodByName(aClass, "setter");
         setter.invoke(instance);
         assertEquals(610, getFoo.invoke(instance));
@@ -247,4 +248,12 @@ public class PropertyGenTest extends CodegenTestCase {
         assertNull("Property should not have a getter", findDeclaredMethodByNameOrNull(c, "getVarNoAccessors"));
         assertNull("Property should not have a setter", findDeclaredMethodByNameOrNull(c, "setVarNoAccessors"));
     }
+
+    // Properties with trivial private setters should not generate their setters since
+    // the class will use direct field access instead for these properties.
+    public void testKt20344() throws Exception {
+        loadText("class Foo { public lateinit var x: String private set }");
+        Class<?> aClass = generateClass("Foo");
+        assertNull("Property should not have generated setter", findDeclaredMethodByNameOrNull(aClass, "setX"));
+    }
 }