Auto merge of #119212 - w-utter:pretty-print-const-expr, r=compiler-errors

Fix representation when printing abstract consts

Previously, when printing a const generic expr, it would only display it as `{{const expr}}`. This allows for a more legible representation when printing these out.

I also zipped the types with their constants for abstract consts that contain function calls when using type annotations, eg: `foo(S: usize, true: bool) -> usize` insteaad of `foo(S, true): fn(usize, bool) -> usize` for conciseness.

Author: bors