Rollup merge of #86176 - nbdd0121:explicit-generic-args, r=jackh726

Implement a `explicit_generic_args_with_impl_trait` feature gate

Implements #83701

When this gate is enabled, explicit generic arguments can be specified even if `impl Trait` is used in argument position. Generic arguments can only be specified for explicit generic parameters but not for the synthetic type parameters from  `impl Trait`

So code like this will be accepted:
```rust
#![feature(explicit_generic_args_with_impl_trait)]

fn foo<T: ?Sized>(_f: impl AsRef<T>) {}
fn main() {
    foo::<str>("".to_string());
}
```

compiler/rustc_infer/src/infer/error_reporting/need_type_info.rs

@@ -769,7 +769,7 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
 
                 // (#83606): Do not emit a suggestion if the parent has an `impl Trait`
                 // as an argument otherwise it will cause the E0282 error.
-                if !has_impl_trait {
+                if !has_impl_trait || self.tcx.features().explicit_generic_args_with_impl_trait {
                     err.span_suggestion_verbose(
                         span,
                         "consider specifying the const argument",