Generate debug information for 'when' subject variable

NB debugger tests currently don't support language version or individual
language feature settings.

compiler/testData/codegen/bytecodeText/when/subjectValInEnumWhenHasLocalVariableSlot.kt

@@ -0,0 +1,12 @@
+// !LANGUAGE: +VariableDeclarationInWhenSubject
+
+enum class X { A, B, C }
+
+fun test(a: X) =
+    when (val subject = a) {
+        X.A -> 0
+        X.B -> 1
+        X.C -> 2
+    }
+
+// 1 LOCALVARIABLE subject