[libc++] Adds tests for LWG-3373.

The code in libc++ already satisfy the requirements of LWG-3373. Since
the issue was written to specifically allow the types to be used in
structured bindings, tests have been added to validate the new
requirement.

Implements
LWG-3373 {to,from}_chars_result and format_to_n_result need the "we really mean what we say" wording

Reviewed By: #libc, ldionne

Differential Revision: https://reviews.llvm.org/D117337