[libc++] [test] Consistently use `REQUIRES: has-unix-headers` (#94122)

There were 7 occurrences of `UNSUPPORTED: !has-unix-headers`, versus 212
occurrences of `REQUIRES: has-unix-headers`.

I don

[libc++] [test] Consistently use `REQUIRES: has-unix-headers` (#94122)

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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[libc++][pstl] Improve exception handling (#88998)

There were various places where we incorrectly handled exceptions in the
PSTL. Typical issues were missing `noexcept` and taking iterators by
val

[libc++][pstl] Improve exception handling (#88998)

There were various places where we incorrectly handled exceptions in the
PSTL. Typical issues were missing `noexcept` and taking iterators by
value instead of by reference.

This patch fixes those inconsistent and incorrect instances, and adds
proper tests for all of those. Note that the previous tests were often
incorrectly turned into no-ops by the compiler due to copy ellision,
which doesn't happen with these new tests.

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