[NFC][libc++] Addresses LWG3903.

Note libc++ actually implemented this wording from the start (D49338).
The Clang version is the same as the version that implements
  P0122R7 <span>

Implements
- LWG3903 span destructor is redundantly noexcept

Reviewed By: #libc, philnik

Differential Revision: https://reviews.llvm.org/D153284