Revert "With the revert of PR 83091, swap is not a const fn anymore."

This reverts commit 34deda3.

Author: usbalbin

src/test/ui/thread-local-static.rs

@@ -11,7 +11,6 @@ const fn g(x: &mut [u32; 8]) {
     //~| ERROR mutable references are not allowed
     //~| ERROR use of mutable static is unsafe
     //~| constant functions cannot refer to statics
-    //~| ERROR calls in constant functions are limited to constant functions
 }
 
 fn main() {}

src/test/ui/thread-local-static.stderr

@@ -30,12 +30,6 @@ LL |     std::mem::swap(x, &mut STATIC_VAR_2)
    = note: see issue #57349 <https://github.com/rust-lang/rust/issues/57349> for more information
    = help: add `#![feature(const_mut_refs)]` to the crate attributes to enable
 
-error[E0015]: calls in constant functions are limited to constant functions, tuple structs and tuple variants
-  --> $DIR/thread-local-static.rs:9:5
-   |
-LL |     std::mem::swap(x, &mut STATIC_VAR_2)
-   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
-
 error[E0133]: use of mutable static is unsafe and requires unsafe function or block
   --> $DIR/thread-local-static.rs:9:23
    |
@@ -44,7 +38,7 @@ LL |     std::mem::swap(x, &mut STATIC_VAR_2)
    |
    = note: mutable statics can be mutated by multiple threads: aliasing violations or data races will cause undefined behavior
 
-error: aborting due to 6 previous errors
+error: aborting due to 5 previous errors
 
-Some errors have detailed explanations: E0013, E0015, E0133, E0658.
+Some errors have detailed explanations: E0013, E0133, E0658.
 For more information about an error, try `rustc --explain E0013`.