Auto merge of #100848 - xfix:use-metadata-for-slice-len, r=thomcc

Use ptr::metadata in <[T]>::len implementation

This avoids duplication of ptr::metadata code.

I believe this is acceptable as the previous approach essentially duplicated `ptr::metadata` because back then `rustc_allow_const_fn_unstable` annotation did not exist.

I would like somebody to ping `@rust-lang/wg-const-eval` as the documentation says:

> Always ping `@rust-lang/wg-const-eval` if you are adding more rustc_allow_const_fn_unstable attributes to any const fn.

Author: bors