When the output buffer is too small to contain the output, `vsnprintf()`
fills the buffer and returns the number of characters that __would have__
been written if the buffer was sufficiently large.
`_vnsprintf_s()` on the other hand fills the buffer and returns -1 when this
happens. We want the former behavior, but we also want to be able to
pass in a locale to prevent having to call `setlocale()`.
`__stdio_common_vsprintf()` is the only function general enough to get
the behavior we want.
Differential Revision: https://reviews.llvm.org/D59727
llvm-svn: 357024