835faa532f
improve type const mismatch errors Doesn't completely remove `check_generic_arg_count` as that would have required some more complex changes but instead checks type and const params in only one step. Also moved the help added by `@JulianKnodt` in #75611 to `generic_arg_mismatch_err`. r? `@varkor` cc `@petrochenkov`
For high-level intro to how type checking works in rustc, see the type checking chapter of the rustc dev guide.