Add regression test for fixed crasher (#20098)

In Swift 4.2, this example crashed. In Swift 5, it incorrectly diagnoses a type error. Eventually, it should actually succeed. For now, let's make sure we don't backslide to crashing.

Author: beccadax

validation-test/compiler_crashers_2_fixed/28872-rdar45583135.swift

@@ -0,0 +1,22 @@
+// This source file is part of the Swift.org open source project
+// Copyright (c) 2014 - 2017 Apple Inc. and the Swift project authors
+// Licensed under Apache License v2.0 with Runtime Library Exception
+//
+// See https://swift.org/LICENSE.txt for license information
+// See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
+
+// RUN: not %target-swift-frontend %s -emit-sil
+
+// One day, if we're lucky, this test will start failing by emitting SIL
+// instead of diagnosing an error. If you are reading this comment because that
+// joyous day has finally arrived, please remove the "not" in the RUN line.
+// Until then, we will at least make sure we don't regress and start crashing
+// again.
+
+func f() -> Int {
+    return { a in
+        { [unowned a] b in
+            a(b)
+        }
+    }(1)
+}