[FIR] Fix IncDecOperatorsInExpectClass for FIR, add multi-module test

- `IncDecOperatorsInExpectClass.kt` should produce an
  `ACTUAL_WITHOUT_EXPECT` in K2 (see KT-55177). This went unnoticed
  because of KT-55570.
- A similar multi-module test for K1 and K2 has been added, called:
  `multiplatform/incDecOperatorsInExpectClass.kt`.

compiler/testData/diagnostics/tests/IncDecOperatorsInExpectClass.fir.kt

@@ -0,0 +1,13 @@
+// In FIR, declaring the same `expect` and `actual` classes in one compiler module is not possible (see KT-55177). Hence the `.fir.kt` test
+// expects a diagnostic here. The multi-module test corresponding to this test is called: `multiplatform/incDecOperatorsInExpectClass.kt`.
+
+// !LANGUAGE: +MultiPlatformProjects
+// SKIP_TXT
+// Issue: KT-49714
+
+expect class Counter {
+    operator fun inc(): Counter
+    operator fun dec(): Counter
+}
+
+<!ACTUAL_WITHOUT_EXPECT!>actual typealias Counter = Int<!>