Resolve variable declaration in 'when' subject

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

@@ -0,0 +1,11 @@
+// !LANGUAGE: +VariableDeclarationInWhenSubject
+// !DIAGNOSTICS: -UNUSED_VARIABLE -UNUSED_PARAMETER
+
+fun foo(): Any = 42
+
+fun test(y: Any) {
+    val z = when (val <!NAME_SHADOWING!>y<!> = foo()) {
+        42 -> "Magic: $y"
+        else -> "Not magic: $y"
+    }
+}