Swift: Add a test for flow through OpenExistentialExpr.

Author: geoffw0

swift/ql/test/library-tests/dataflow/dataflow/LocalFlow.expected

@@ -924,3 +924,5 @@
 | test.swift:766:9:766:9 | f | test.swift:766:9:766:9 | SSA def(f) |
 | test.swift:766:13:766:29 | #keyPath(...) | test.swift:766:9:766:9 | f |
 | test.swift:766:13:766:29 | enter #keyPath(...) | test.swift:766:26:766:26 | KeyPathComponent |
+| test.swift:774:30:774:33 | SSA def(x) | test.swift:775:12:775:12 | x |
+| test.swift:774:30:774:33 | x | test.swift:774:30:774:33 | SSA def(x) |

swift/ql/test/library-tests/dataflow/dataflow/test.swift

@@ -766,3 +766,11 @@ func testOptionalKeyPathForce() {
     let f = \S2_Optional.s!.x
     sink(arg: s2[keyPath: f]) // $ flow=764
 }
+
+protocol MyProtocol {
+	func source() -> Int
+}
+
+func testOpenExistentialExpr(x: MyProtocol) {
+	sink(arg: x.source()) // $ MISSING:flow=771
+}