rollup merge of #21495: richo/unexported-unmangled-lint

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.

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