[WIP] Add Compiler Smoke Tests for (suspend) main methods

This commit ports the (parameterless) main integration tests in
`CompilerSmokeTest` to the IR backend. It also includes a simple
suspend main test.

The advanced ones (like `helloAppSuspendParameterlessMain`) are
currently blocked by pending changes to capturing suspend lambdas,
which are underway.

compiler/testData/integration/smoke/helloAppVarargMainIR/hello.kt

@@ -0,0 +1,6 @@
+package Hello
+
+fun main(vararg args: kotlin.String) {
+    args.size
+    System.out.println("Hello from vararg main!")
+}