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/subjectValHasLocalVariableSlot.kt

@@ -0,0 +1,10 @@
+// !LANGUAGE: +VariableDeclarationInWhenSubject
+
+fun test(a: Any) =
+    when (val subject = a) {
+        is Int -> "Int $subject"
+        is String -> "String ${subject.length}"
+        else -> "other"
+    }
+
+// 1 LOCALVARIABLE subject Ljava/lang/Object;