[patch] Fix codecvt<char16_t, char, mbstate_t>

[Jonathan Wakely]

While working on PR64797 I discovered that the codecvt<char16_t,...>
specialization was, erm, completely broken when creating UTF-16
surrogate pairs.

This fixes it and adds a test, based on the char32_t one I added to
the testsuite yesterday. Tested x86_64-linux (little-endian) and
powerpc64-linux (big-endian).


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