incr.comp.: Implement query diagnostic persistence.

src/test/incremental/warnings-reemitted.rs

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+// Copyright 2017 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// revisions: cfail1 cfail2 cfail3
+// compile-flags: -Coverflow-checks=on
+// must-compile-successfully
+
+#![allow(warnings)]
+
+fn main() {
+    255u8 + 1; //~ WARNING this expression will panic at run-time
+}