[libcxx] Fix the error checking for wctob_l, fixing locale narrow function on Windows

[libcxx] Fix the error checking for wctob_l, fixing locale narrow function on Windows

According to POSIX.1 (and Glibc docs, and Microsoft docs), the wctob
function returns EOF on error, not WEOF. (And wctob_l should consequently
do the same.)

The previous misconception about what this function returns on errors
seems to stem from incorrect documentation in macOS, stemming from BSD
docs with the same issue. The corresponding documentation bug in FreeBSD
was fixed in 2012 in
https://github.com/freebsd/freebsd-src/commit/945aab90991bdaeabeb6ef25112975a96c01dd4e,
but it hasn't been fixed for macOS yet.

The issue seems to only be a documentation issue; the implementation
on macOS actually does use EOF, not WEOF:
https://opensource.apple.com/source/Libc/Libc-1439.40.11/locale/FreeBSD/wctob.c.auto.html

On most Unices, EOF and WEOF are the same value, but on Windows,
EOF is -1, while WEOF is (unsigned short)0xFFFF. By fixing this,
two tests start passing on Windows.

Differential Revision: https://reviews.llvm.org/D120088