Support main entry-point without arguments in frontend

#KT-26574 In Progress

compiler/testData/diagnostics/tests/redeclarations/RedeclarationParameterlessMain.kt

@@ -0,0 +1,5 @@
+// FILE: a.kt
+fun main() {}
+
+// FILE: b.kt
+fun main() {}

compiler/testData/diagnostics/tests/redeclarations/RedeclarationParameterlessMain.txt

@@ -0,0 +1,4 @@
+package
+
+public fun main(): kotlin.Unit
+public fun main(): kotlin.Unit

compiler/testData/diagnostics/tests/redeclarations/RedeclarationParameterlessMainInvalid.kt

@@ -0,0 +1,7 @@
+// FILE: a.kt
+<!CONFLICTING_OVERLOADS!>fun main()<!> {}
+
+suspend fun main(args: Array<String>) {}
+
+// FILE: b.kt
+<!CONFLICTING_OVERLOADS!>fun main()<!> {}

compiler/testData/diagnostics/tests/redeclarations/RedeclarationParameterlessMainInvalid.txt

@@ -0,0 +1,5 @@
+package
+
+public fun main(): kotlin.Unit
+public fun main(): kotlin.Unit
+public suspend fun main(/*0*/ args: kotlin.Array<kotlin.String>): kotlin.Unit