Auto merge of #21495 - richo:unexported-unmangled-lint, r=alexcrichton

The usecase is that functions made visible to systems outside of the
rust ecosystem require the symbol to be visible.

This adds a lint for functions that are not exported, but also not mangled.

It has some gotchas:

[ ]: There is fallout in core that needs taking care of
[ ]: I'm not convinced the error message is correct
[ ]: It has no tests

~~However, there's an underlying issue which I'd like feedback on- which is that my belief that that non-pub functions would not have their symbols exported, however that seems not to be the case in the first case that this lint turned up in rustc (`rust_fail`), which intuition suggests has been working.~~

This seems to be a separate bug in rust, wherein the symbols are exported in binaries, but not in rlibs or dylibs. This lint would catch that case.

src/test/compile-fail/lint-unexported-no-mangle.rs

@@ -0,0 +1,25 @@
+// Copyright 2015 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.
+
+// compile-flags:-F private_no_mangle_fns
+
+// FIXME(#19495) no_mangle'ing main ICE's.
+#[no_mangle]
+fn foo() { //~ ERROR function foo is marked #[no_mangle], but not exported
+}
+
+#[no_mangle]
+pub fn bar()  {
+}
+
+fn main() {
+    foo();
+    bar();
+}