Add test for VAL_REASSIGNMENT on when subject variable

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

@@ -0,0 +1,10 @@
+// !LANGUAGE: +VariableDeclarationInWhenSubject
+// !DIAGNOSTICS: -UNUSED_VARIABLE -UNUSED_PARAMETER -UNUSED_VALUE
+
+fun foo(): Any = 42
+
+fun test(x: Any) {
+    when (val y = foo()) {
+        is String -> <!VAL_REASSIGNMENT!>y<!> = ""
+    }
+}