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_span/src/symbol.rs

@@ -554,6 +554,7 @@ symbols! {
         expected,
         expf32,
         expf64,
+        explicit_generic_args_with_impl_trait,
         export_name,
         expr,
         extended_key_value_attributes,