Rollup merge of #127622 - compiler-errors:builtin-internal, r=lqd

Mark `builtin_syntax` as internal
Tracking issue literally says:
> There will never be a general stabilization.
cc #110680 `@est31`

Author: matthiaskrgr

compiler/rustc_feature/src/unstable.rs

@@ -248,6 +248,8 @@ declare_features! (
 (unstable, auto_traits,"1.50.0",Some(13231)),
 /// Allows using `box` in patterns (RFC 469).
 (unstable, box_patterns,"1.0.0",Some(29641)),
+/// Allows builtin # foo() syntax
+(internal, builtin_syntax,"1.71.0",Some(110680)),
 /// Allows `#[doc(notable_trait)]`.
 /// Renamed from `doc_spotlight`.
 (unstable, doc_notable_trait,"1.52.0",Some(45040)),
@@ -361,8 +363,6 @@ declare_features! (
 (unstable, async_fn_track_caller,"1.73.0",Some(110011)),
 /// Allows `for await` loops.
 (unstable, async_for_loop,"1.77.0",Some(118898)),
-/// Allows builtin # foo() syntax
-(unstable, builtin_syntax,"1.71.0",Some(110680)),
 /// Allows using C-variadics.
 (unstable, c_variadic,"1.34.0",Some(44930)),
 /// Allows the use of `#[cfg(overflow_checks)` to check if integer overflow behaviour.