[RFC] Add flag for Unicode based Frontend support

[estebank]

Using PR #21406 as a base, I took a stab at #24387.

It currently looks like this:

As you can see, there's still some work left to do in order for the different combinations of underlines to look right, but the basic logic is already all there.

[rust-highfive]

r? @eddyb

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[frewsxcv]

Travis encountered some compile errors

[eddyb]

r? @jonathandturner

[sophiajt]

Unfortunately, while this helps with some platforms, other platforms like Windows can't experience it. In the RFC for the new error format, we call out that Unicode should not be supported: https://github.com/rust-lang/rfcs/blob/master/text/1644-default-and-expanded-rustc-errors.md#format-of-error-messages

That said, there are probably ways we can improve some of our ascii drawing for better readability in some cases, like the multiline case.

[bors]

☔ The latest upstream changes (presumably #37681) made this pull request unmergeable. Please resolve the merge conflicts.

[steveklabnik]

@jonathandturner I'm going to take that comment as an "this should be closed", then. Let me know if that's wrong!

Thanks @estebank but we can't accept this patch.