Set loop index variable if not "_" (#3)

Signed-off-by: Anand Krishnamoorthi <anakrish@microsoft.com>

tests/interpreter/cases/loop/basic.yaml

@@ -0,0 +1,23 @@
+# Copyright (c) Microsoft Corporation.
+# Licensed under the MIT License.
+cases:
+  - note: basic
+    data: {}
+    modules:
+      - |
+        package test
+
+        x1 = y {
+          y = [ [a, b] | a = [1, 2, 3, 4][b] ]
+        }
+        x2 = y {
+          y = [ [a, b] | a = {1, 2, 3, 4}[b] ]
+        }
+        x3 = y {
+          y = [ [a, b] | a = {"p":"q", "r": "s"}[b] ]
+        }
+    query: data.test
+    want_result:
+      x1: [[1, 0], [2, 1], [3, 2], [4, 3]]
+      x2: [[1, 1], [2, 2], [3, 3], [4, 4]]
+      x3: [["q", "p"], ["s", "r"]]