[JVM IR] Fix JvmOverloads+Parameterless Main

Resolves the interaction of @JvmOverloads annotations and
parameterless main methods.

In the following code, both mechanisms generate methods that
ultimately produce the signature `public static void main(String[] args)`
of which there can be only one (true in general of any signature).

```
fun main() { }

@JvmOverloads
fun main(Array<String> args, x: Int = 42) { }
```

This PR simply shuffles the lowerings around, letting parameterless
main methods detect the presence of the default overload produced by
the annotation.

Additionally, this PR improves the testing of parameterless main
methods by actual bytecode patterns, and not simple check for
successful compilation (as @sfs and I discovered, there are issues in
flagging an error on duplicate signatures on the IR backend).

compiler/testData/codegen/bytecodeText/parameterlessMain/dontGenerateOnVarargsString.kt

@@ -0,0 +1,9 @@
+fun main() {
+
+}
+
+fun main(vararg args: String) {
+
+}
+
+// 0 INVOKESTATIC DontGenerateOnVarargsStringKt\.main ()V