Update the coherence rules to "covered first"

[nikomatsakis]

Update the coherence rules to "covered first" -- the first type parameter to contain either a local type or a type parameter must contain only covered type parameters.

cc #19470.
Fixes #20974.
Fixes #20749.

r? @aturon

[nikomatsakis]

Still running make check locally.

[nikomatsakis]

OK, tweaked a few tests, should pass make check now (though the last few bits are still running).

[aturon]

Looks good. One question: presumably the error messages need to be updated to clarify the new rule? Unless they were vague enough before that they accurately describe the new rule as well?

[nikomatsakis]

@aturon I felt they were vague enough to still be accurate, but I'm open to suggestions.

[nikomatsakis]

Here are the messages in question:

https://github.com/nikomatsakis/rust/blob/ecaa68768cdc1fa82a750998634e9cc3202975f9/src/librustc_typeck/coherence/orphan.rs#L76-L104

[nikomatsakis]

Well, you were correct that one of them at least was outdated. (In fact, I had fixed that message, but as part of a commit removing old-orphan-check that I was forced to back out.) Good catch!

[nikomatsakis]

@bors r=aturon 2c2879b

[nikomatsakis]

@aturon I interpreted that as r+ and decided to take advantage of a quiet moment in the bors queue. If we want to tweak error messages further we can always do so.

[bors]

⌛ Testing commit 2c2879b with merge e8489d3...

[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