Convert `check_neg_trait_impl` to a judgment function

[nikomatsakis]

Parent tracking issue: #288

Convert check_neg_trait_impl from a regular Rust function to a judgment function.

Currently it: opens the binder, proves WF + not_implemented, and validates that negative impls are not declared unsafe.

See the parent tracking issue for context and examples.

This issue has been assigned to @Mslady-jojo via this comment.

[dinahmaccodes]

@rustbot claim

[nikomatsakis]

I'm going to go ahead and remove the assignment here as I do not believe @dinahmaccodes is still active.

@TriageBot release-assignment

If you are still interested in working on a-mir-formality, @dinahmaccodes, please come by the rust-lang zulip and let's chat!

[nikomatsakis]

@rustbot assign @JojoFlex1

[nikomatsakis]

@rustbot assign @Mslady-jojo