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.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@188077 91177308-0d34-0410-b5e6-96231b3b80d8