PR22208: On FreeBSD systems, __STDC_MB_MIGHT_NEQ_WC__ is expected to be defined

even though every basic source character literal has the same numerical value
as a narrow or wide character literal.

It appears that the FreeBSD folks are trying to use this macro to mean
something other than what the relevant standards say it means, but their usage
is conforming, so put up with it.

llvm-svn: 225751