Implement a explicit_generic_args_with_impl_trait feature gate

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`

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,