There were 7 occurrences of `UNSUPPORTED: !has-unix-headers`, versus 212
occurrences of `REQUIRES: has-unix-headers`.
I don't completely understand how libc++ uses UNSUPPORTED versus
REQUIRES, but it seems better to be consistent, and to avoid the double
negation in "this is unsupported if we don't have unix headers".
(This came to my attention because of the single occurrence in
`libcxx/test/std`. Our MSVC-internal test harness isn't aware of lit
features, so we teach it to skip tests via the incredibly primitive
method of searching for specific comments, so I had to deal with this
comment inconsistency.)
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