[PATCH] Various fixes for <codecvt> facets

[dawn.on.the.edge]

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.)

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