Rollup merge of #152749 - cyrgani:rustc-allow-const, r=jdonszelmann

make `rustc_allow_const_fn_unstable` an actual `rustc_attrs` attribute

It is already named like one, but used to have its own feature gate, which this PR now removes in favor of just using `#![feature(rustc_attrs)]`.

Most of the diff is just the line number changes in `malformed-attrs.stderr`.

library/alloc/src/lib.rs

@@ -182,7 +182,6 @@
 #![feature(negative_impls)]
 #![feature(never_type)]
 #![feature(optimize_attribute)]
-#![feature(rustc_allow_const_fn_unstable)]
 #![feature(rustc_attrs)]
 #![feature(slice_internals)]
 #![feature(staged_api)]

library/core/src/lib.rs

@@ -159,7 +159,6 @@
 #![feature(pattern_types)]
 #![feature(prelude_import)]
 #![feature(repr_simd)]
-#![feature(rustc_allow_const_fn_unstable)]
 #![feature(rustc_attrs)]
 #![feature(rustdoc_internals)]
 #![feature(simd_ffi)]