Add some data flow analysis tests on when

compiler/testData/diagnostics/tests/when/withSubjectVariable/noSubjectVariableName.kt

@@ -0,0 +1,10 @@
+// !LANGUAGE: +VariableDeclarationInWhenSubject
+// !DIAGNOSTICS: -UNUSED_VARIABLE -UNUSED_PARAMETER
+
+
+fun testNoSubjectVariableName(x: Int?) {
+    val y = when (val<!SYNTAX!><!> = 42) {
+        0 -> "0"
+        else -> "not 0"
+    }
+}