Remove the try/catch codepath if `swap` is `noexcept`.

Summary:
In the case where `swap` is `noexcept`, we should avoid the extension to provide strong-exception guarantee.

Fixes https://bugs.llvm.org/show_bug.cgi?id=46342

Reviewers: #libc, ldionne

Reviewed By: #libc, ldionne

Subscribers: dexonsmith, mclow.lists, miscco, ldionne, zoecarver, libcxx-commits

Tags: #libc

Differential Revision: https://reviews.llvm.org/D81954