[libc] Add support for wchar_t in StringConverter. (#211867)

This takes care of a TODO in LibcTest.cpp from #203355, allowing
`wstring_view` to be printed as UTF-8 in tests. It also enables updates
to __support/printf_core/ for implementing `swprintf`.

The change to `StringConverter` is enabled by extending the
`CharacterConverter` API when `wchar_t` is detected as being UTF-32.

The `StringConverter` unit test is updated so it no longer compiles to a
no-op on Windows.

Note: this change does not respect `-fwide-exec-charset`, which Clang still
does not support but GCC does. This is the existing libc behavior.
GitOrigin-RevId: e84b8cf804ea41c7ed1c1c64c519b6355abafa31