This should make it easier to get better output when wchar_t tests fail.
The code is based on the Unicode transcoding in `<format>`.
Differential Revision: https://reviews.llvm.org/D150593
* Silence warnings for unused parameters / variables
* There's no reason to output unexpected exceptions only for libc++
Differential Revision: https://reviews.llvm.org/D141453
Update the formatter day tests to the new style.
Other test will be done separately.
Reviewed By: #libc, ldionne
Differential Revision: https://reviews.llvm.org/D134031