[PATCH] Various fixes for <codecvt> facets

[Jonathan Wakely]

On 17/03/17 14:01 -0700, dawn.on.the.edge wrote:
>I’m glad to see someone's on this already, because I had just noticed a major bug in codecvt_16<wchar_t>: it converts to UTF-8, not UTF-16.
>
>You might have gotten to it already, but I’ve included a testcase that reproduces the bug.  It passes with clang -std=c++14 -stdlib=libc++ but fails with either clang -std=c++14 -stdilb=libstdc++ or g++ -std=c++14.  (And it needs some extra magic to work on MSVC.)

That was https://gcc.gnu.org/bugzilla/show_bug.cgi?id=80041 and is now
fixed.