JVM IR: Support parameterless main methods

This commit:

- introduces tests explicating what is and isn't considered a
  proper main method on the JVM backends.
- implements support for parameterless main methods on the JVM IR
  backend
- See KT-34338 for more tests.

compiler/testData/writeSignature/parameterlessMain/arrayWithContravariantStringIsNotMainMethod.kt

@@ -0,0 +1,11 @@
+fun main() {
+
+}
+
+fun main(args: Array<in String>) {
+
+}
+
+// method: ArrayWithContravariantStringIsNotMainMethodKt::main
+// jvm signature:     ([Ljava/lang/String;)V
+// generic signature: null

compiler/testData/writeSignature/parameterlessMain/impostorMains.kt

@@ -0,0 +1,24 @@
+fun main() {
+
+}
+
+fun main(args: Array<Int>) {
+
+}
+
+fun main(args: Array<in String>) {
+
+}
+
+fun main(args: Array<String>): Unit? {
+
+    return Unit
+}
+
+fun Any.main(args: Array<String>) {
+
+}
+
+// method: ImpostorMainsKt::main
+// jvm signature:     ([Ljava/lang/String;)V
+// generic signature: null

compiler/testData/writeSignature/parameterlessMain/main.kt

@@ -0,0 +1,7 @@
+fun main(args: Array<String>) {
+
+}
+
+// method: MainKt::main
+// jvm signature:     ([Ljava/lang/String;)V
+// generic signature: null

compiler/testData/writeSignature/parameterlessMain/notJvmMainName.kt

@@ -0,0 +1,21 @@
+// WITH_RUNTIME
+fun main() {
+
+}
+
+@JvmName("notMain")
+fun main(args: Array<String>) {
+
+}
+
+// method: NotJvmMainNameKt::main
+// jvm signature:     ()V
+// generic signature: null
+
+// method: NotJvmMainNameKt::main
+// jvm signature:     ([Ljava/lang/String;)V
+// generic signature: null
+
+// method: NotJvmMainNameKt::notMain
+// jvm signature:     ([Ljava/lang/String;)V
+// generic signature: null

compiler/testData/writeSignature/parameterlessMain/simple.kt

@@ -0,0 +1,11 @@
+fun main() {
+
+}
+
+// method: SimpleKt::main
+// jvm signature:     ()V
+// generic signature: null
+
+// method: SimpleKt::main
+// jvm signature:     ([Ljava/lang/String;)V
+// generic signature: null