fix main function detection to rely on actually resolved types

compiler/integration-tests/testData/compileAndRunHelloAppVarargMain/hello.compile.expected

@@ -0,0 +1 @@
+Return code: 0

compiler/integration-tests/testData/compileAndRunHelloAppVarargMain/hello.kt

@@ -0,0 +1,5 @@
+package Hello
+
+fun main(vararg args: kotlin.String) {
+    System.out.println("Hello from vararg main!")
+}

compiler/integration-tests/testData/compileAndRunHelloAppVarargMain/hello.run.expected

@@ -0,0 +1,4 @@
+OUT:
+Hello from vararg main!
+
+Return code: 0