Add long diagnostics for enum repr errors.

[michaelsproul]

Explanations for E0079, E0080, E0081, E0082, E0083 and E0084 as part of #24407.

All the errors concern the use of #[repr(X)] with enum types.

I also updated the short description for E0079 so that it takes sign into account.

[rust-highfive]

r? @Aatch

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[michaelsproul]

Here's an example where the current error for E0079 is misleading.

#[repr(u8)]
enum Wow {
    X = "what",
    Y
}

fn main() {}

what.rs:3:9: 3:15 error: mismatched types:
 expected `u8`,
    found `&'static str`
(expected u8,
    found &-ptr) [E0308]
what.rs:3     X = "what",
                  ^~~~~~
what.rs:3:9: 3:15 error: expected signed integer constant [E0079]
what.rs:3     X = "what",
                  ^~~~~~
error: aborting due to 2 previous errors

Note the mention of signed integer constant.

[michaelsproul]

make tidy is broken because of errorck.py, which was made redundant by the uniqueness checking I added in #24884.

I think errorck.py could be safely deleted unless someone has further plans for it? @brson?

[pnkfelix]

@michaelsproul I think errorck.py still remains useful as a quick way to find the highest error code, no?

In the short term, maybe you could work around it by changing the text to say "error code 0082" .. though I can see how that is not ideal.

Nonetheless, I'm not 100% sure that referring users to another error code is really great practice... would it not be better to tell users to look up the #[repr(..)] attribute in the documentation?

[michaelsproul]

@pnkfelix: You're absolutely right! I'll leave errock.py alone and change my explanation to point to the reference.

[pnkfelix]

@bors r+ 63e6321 rollup

[bors]

⌛ Testing commit 63e6321 with merge e962870...

[bors]

☀️ Test successful - auto-linux-32-nopt-t, auto-linux-32-opt, auto-linux-64-nopt-t, auto-linux-64-opt, auto-linux-64-x-android-t, auto-mac-32-opt, auto-mac-64-nopt-t, auto-mac-64-opt, auto-win-32-nopt-t, auto-win-32-opt, auto-win-64-nopt-t, auto-win-64-opt