QL: Add test for override

This test demonstrates that our handling of `override` is incorrect.

Quick-eval'ing the `test` predicate produces the following output:

| f |  i  |  j  |
+---+-----+-----+
| 1 |  10 |  10 |
| 1 |  10 | 100 |
| 1 | 100 |  10 |
| 1 | 100 | 100 |
| 2 |  20 |  20 |
| 3 |   3 |   3 |

this demonstrates that `f.bar` and `f.baz` can resolve to all predicates
of that name in the file.

However, at present we only capture the calls to members on `Foo`.

Author: tausbn

ql/test/callgraph/Overrides.qll

@@ -0,0 +1,30 @@
+import ql
+
+class Foo extends int {
+  Foo() { this in [1, 2, 3] }
+
+  int bar() { result = this }
+
+  predicate baz(int i) { i = this.bar() }
+}
+
+class Bar extends Foo {
+  Bar() { this = [1, 2] }
+
+  override int bar() { result = 10 * this }
+
+  override predicate baz(int i) { i = this.bar() }
+}
+
+class Baz extends Foo {
+  Baz() { this = 1 }
+
+  override int bar() { result = 100 * this }
+
+  override predicate baz(int i) { i = this.bar() }
+}
+
+query predicate test(Foo f, int i, int j) {
+  f.bar() = i and
+  f.baz(j)
+}

ql/test/callgraph/callgraph.expected

@@ -6,3 +6,10 @@
 | Foo.qll:29:5:29:16 | PredicateCall | Foo.qll:26:3:26:32 | ClasslessPredicate alias2 |
 | Foo.qll:31:5:31:12 | PredicateCall | Foo.qll:22:3:22:32 | ClasslessPredicate myThing0 |
 | Foo.qll:31:5:31:12 | PredicateCall | Foo.qll:24:3:24:32 | ClasslessPredicate alias0 |
+| Overrides.qll:8:30:8:39 | MemberCall | Overrides.qll:6:3:6:29 | ClassPredicate bar |
+| Overrides.qll:16:39:16:48 | MemberCall | Overrides.qll:6:3:6:29 | ClassPredicate bar |
+| Overrides.qll:16:39:16:48 | MemberCall | Overrides.qll:14:12:14:43 | ClassPredicate bar |
+| Overrides.qll:24:39:24:48 | MemberCall | Overrides.qll:6:3:6:29 | ClassPredicate bar |
+| Overrides.qll:24:39:24:48 | MemberCall | Overrides.qll:22:12:22:44 | ClassPredicate bar |
+| Overrides.qll:28:3:28:9 | MemberCall | Overrides.qll:6:3:6:29 | ClassPredicate bar |
+| Overrides.qll:29:3:29:10 | MemberCall | Overrides.qll:8:3:8:41 | ClassPredicate baz |