Resolve variable declaration in 'when' subject

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

@@ -0,0 +1,14 @@
+// !LANGUAGE: +VariableDeclarationInWhenSubject
+// !DIAGNOSTICS: -UNUSED_VARIABLE
+
+fun foo(): Any = 42
+
+fun test(x: Any) {
+    val z1 = when (val y = foo()) {
+        42 -> "Magic: $y, $x"
+        else -> {
+            "Not magic: $y, $x"
+        }
+    }
+    val z2 = "Anyway, it was $<!UNRESOLVED_REFERENCE!>y<!>"
+}