FIR: introduce Java vs Kotlin class conflicts checker #KT-44441 Fixed

compiler/testData/diagnostics/tests/j+k/brokenCode/classDuplicates.fir.kt

@@ -0,0 +1,13 @@
+// FILE: Hello.kt
+private class <!PACKAGE_OR_CLASSIFIER_REDECLARATION!>Hello<!>()
+{
+    val a = 4
+}
+
+fun test() {
+    // no exception is thrown (see KT-3897)
+    Hello().a
+}
+
+// FILE: Hello.java
+public class Hello {}