Rollup merge of #110791 - compiler-errors:negative-bounds, r=oli-obk

Implement negative bounds for internal testing purposes

Implements partial support the `!` negative polarity on trait bounds. This is incomplete, but should allow us to at least be able to play with the feature.

Not even gonna consider them as a public-facing feature, but I'm implementing them because would've been nice to have in UI tests, for example in #110671.

compiler/rustc_middle/src/ty/print/pretty.rs

@@ -2816,6 +2816,9 @@ define_print_and_forward_display! {
         if let ty::BoundConstness::ConstIfConst = self.constness && cx.tcx().features().const_trait_impl {
             p!("~const ");
         }
+        if let ty::ImplPolarity::Negative = self.polarity {
+            p!("!");
+        }
         p!(print(self.trait_ref.print_only_trait_path()))
     }