Revert "Remove #[alloc_error_handler] from the compiler and library"

This reverts commit abc0660118.

compiler/rustc_feature/src/active.rs

@@ -291,6 +291,8 @@ declare_features! (
     (active, abi_x86_interrupt, "1.17.0", Some(40180), None),
     /// Allows additional const parameter types, such as `&'static str` or user defined types
     (incomplete, adt_const_params, "1.56.0", Some(95174), None),
+    /// Allows defining an `#[alloc_error_handler]`.
+    (active, alloc_error_handler, "1.29.0", Some(51540), None),
     /// Allows trait methods with arbitrary self types.
     (active, arbitrary_self_types, "1.23.0", Some(44874), None),
     /// Allows using `const` operands in inline assembly.