Partial implementation of N3665. This paper was not voted into the C++1y draft. However I was looking at it and with some experimentation realized that I could partially implement it, and at the same time offer a performance optimization to cout. I simply added an xsputn override to the cout filebuf. The override does nothing special at all if there is a non-trivial codecvt installed. However if the codecvt returns true for always_noconv(), then this function can dump an entire string to fwrite, instead of doing it a character at a time under overflow(). This just makes sense. I stopped short of a full implementation of N3665 because in order to do so, xsputn would have to allocate a buffer when always_noconv() returned false, and I don't want to go to that expense.

llvm-svn: 188077

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