Make $default methods non-private, avoid generating accessors for them

#KT-5786 Fixed

compiler/testData/codegen/box/defaultArguments/private/memberExtensionFunction.kt

@@ -0,0 +1,9 @@
+class A {
+    private fun Int.foo(other: Int = 5): Int = this + other
+
+    inner class B {
+        fun bar() = 37.foo()
+    }
+}
+
+fun box() = if (A().B().bar() == 42) "OK" else "Fail"

compiler/testData/codegen/box/defaultArguments/private/memberFunction.kt

@@ -0,0 +1,11 @@
+// KT-5786 NoSuchMethodError: no accessor for private fun with default arguments
+
+class A {
+    private fun foo(result: String = "OK"): String = result
+
+    companion object {
+        fun bar() = A().foo()
+    }
+}
+
+fun box() = A.bar()

compiler/testData/codegen/box/defaultArguments/private/primaryConstructor.kt

@@ -0,0 +1,16 @@
+var state: String = "Fail"
+
+class A private(x: String = "OK") {
+    init {
+        state = x
+    }
+
+    companion object {
+        fun foo() = A()
+    }
+}
+
+fun box(): String {
+    A.foo()
+    return state
+}

compiler/testData/codegen/box/defaultArguments/private/secondaryConstructor.kt

@@ -0,0 +1,16 @@
+var state: String = "Fail"
+
+class A {
+    private constructor(x: String = "OK") {
+        state = x
+    }
+
+    companion object {
+        fun foo() = A()
+    }
+}
+
+fun box(): String {
+    A.foo()
+    return state
+}

compiler/testData/codegen/boxWithStdlib/regressions/kt5786_privateWithDefault.kt

@@ -0,0 +1,12 @@
+fun box(): String {
+    run {
+        test("ok")
+        test("ok", 200)
+    }
+    test("ok")
+    test("ok", 300)
+
+    return "OK"
+}
+
+private fun test(arg1: String, default: Int = 0) = Unit