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 - rust-lang/rfcs#3530.

Author: petrochenkov

compiler/rustc_feature/src/unstable.rs

@@ -456,6 +456,8 @@ declare_features! (
     (unstable, ffi_returns_twice, "1.34.0", Some(58314), None),
     /// Allows using `#[repr(align(...))]` on function items
     (unstable, fn_align, "1.53.0", Some(82232), None),
+    /// Support delegating implementation of functions to other already implemented functions.
+    (incomplete, fn_delegation, "CURRENT_RUSTC_VERSION", Some(118212), None),
     /// Allows defining gen blocks and `gen fn`.
     (unstable, gen_blocks, "1.75.0", Some(117078), None),
     /// Infer generic args for both consts and types.

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,

tests/ui/feature-gates/feature-gate-fn_delegation.rs

@@ -0,0 +1,3 @@
+todo!(); //~ ERROR
+
+fn main() {}

tests/ui/feature-gates/feature-gate-fn_delegation.stderr

@@ -0,0 +1,13 @@
+error: expected one of `!` or `::`, found `(`
+  --> $DIR/feature-gate-fn_delegation.rs:1:1
+   |
+LL | todo!();
+   | ^^^^^^^
+   | |
+   | expected one of `!` or `::`
+   | in this macro invocation
+   |
+   = note: this error originates in the macro `todo` (in Nightly builds, run with -Z macro-backtrace for more info)
+
+error: aborting due to previous error
+