Add test for smart cast on a value bound to when subject variable

E.g.:
  when (val y = x?.foo()) {
    null -> ...
    else -> ... // 'x' is also non-null here
  }

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

@@ -0,0 +1,9 @@
+// !LANGUAGE: +VariableDeclarationInWhenSubject
+
+fun foo(s1: String, s2: String) = s1 + s2
+
+fun test(x: Any?) =
+        when (val y = x?.toString()) {
+            null -> "null"
+            else -> foo(<!DEBUG_INFO_SMARTCAST!>x<!>.toString(), <!DEBUG_INFO_SMARTCAST!>y<!>)
+        }

compiler/testData/diagnostics/tests/when/withSubjectVariable/smartCastOnValueBoundToSubjectVariable.txt

@@ -0,0 +1,4 @@
+package
+
+public fun foo(/*0*/ s1: kotlin.String, /*1*/ s2: kotlin.String): kotlin.String
+public fun test(/*0*/ x: kotlin.Any?): kotlin.String