Auto merge of #24975 - michaelsproul:enum-diagnostics, r=pnkfelix

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.

diff --cc src/librustc/diagnostics.rs
Simple merge