Add an experimental feature gate for function delegation

In accordance with the [process](https://github.com/rust-lang/lang-team/blob/master/src/how_to/experiment.md).

Detailed description of the feature can be found in the RFC repo - https://github.com/rust-lang/rfcs/pull/3530.

compiler/rustc_span/src/symbol.rs

@@ -787,6 +787,7 @@ symbols! {
         fmt,
         fmul_fast,
         fn_align,
+        fn_delegation,
         fn_must_use,
         fn_mut,
         fn_once,