html output break utf-8

[Dushistov]

I have lines like this in my codebase:

 if !matches!(it.next(), Some('A'..='Z') | Some('А'..='Я')) {
        return false;
 }

Generating html with such command cargo llvm-cov --html test.

For letter 'Я' resulted html code looks like this:

=&apos;\320<span class='tooltip-content'>5</span></div>\257&apos;

so html generator breaks valid utf-8 sequence for 'Я' (\320\257) into two parts and insert html code between,
making it invalid.

[Zalathar]

This should now be fixed upstream in nightly as of rust-lang/rust#119033.

[taiki-e]

@Zalathar Great! Thanks for fixing this!