Add test that started crashing with my initial rework of closure discriminators.

Thanks to Alex Hoppen for providing this test!

Author: DougGregor

test/SourceKit/CodeComplete/crash_discriminator.swift

@@ -0,0 +1,8 @@
+func foo(array: [Int]) {
+    _ = array.map { (value) -> Int in
+// RUN: %sourcekitd-test -req=complete -pos=%(line+1):20 %s -- %s == -req=complete -pos=%(line+2):16 %s -- %s
+        let decoded = 42
+        return decoded
+    }
+}
+