[JVM_IR] Fix mangling of default argument stubs for internal methods.

The MethodSignatureMapper expected to be able to look at the body
of the default argument stub. That is of course not possible when
it is from an external dependency.

Instead, we go through the attribute owner to get to the method
the stub is a default argument adapter for.

compiler/testData/compileKotlinAgainstKotlin/internalWithDefaultArgs.kt

@@ -0,0 +1,15 @@
+// IGNORE_BACKEND_FIR: JVM_IR
+// IGNORE_BACKEND: NATIVE
+// FILE: A.kt
+
+package a
+
+class Box() {
+    internal fun result(value: String = "OK"): String = value
+}
+
+// FILE: B.kt
+
+fun box(): String {
+    return a.Box().result()
+}