[libc++][test] Fix issues found by MSVC's STL (#131787)

* libcxx/test/support/min_allocator.h
+ Fix `tiny_size_allocator::rebind` which mistakenly said `T` instead of
`U`.
*
libcxx/test/std/algorithms/alg.modifying.operations/alg.partitions/stable_partition.pass.cpp
  + `std::stable_partition` requires bidirectional iterators.
* libcxx/test/std/containers/sequences/vector.bool/max_size.pass.cpp
+ Fix allocator type given to `std::vector<bool>`. The element types are
required to match, [N5008](https://isocpp.org/files/papers/N5008.pdf)
\[container.alloc.reqmts\]/5: "*Mandates:* `allocator_type::value_type`
is the same as `X::value_type`."
* libcxx/test/std/time/time.clock/time.clock.utc/types.compile.pass.cpp
+ Mark `is_steady` as `[[maybe_unused]]`, as it appears within
`LIBCPP_STATIC_ASSERT` only.
*
libcxx/test/std/algorithms/alg.modifying.operations/alg.rotate/rotate.pass.cpp
*
libcxx/test/std/algorithms/alg.modifying.operations/alg.swap/swap_ranges.pass.cpp
* libcxx/test/std/utilities/utility/utility.swap/swap_array.pass.cpp
+ Fix MSVC warning C4127 "conditional expression is constant".
`TEST_STD_AT_LEAST_23_OR_RUNTIME_EVALUATED` was introduced for this
purpose, so it should be used consistently.
* libcxx/test/std/numerics/numeric.ops/numeric.ops.gcd/gcd.pass.cpp
+ Fix `gcd()` precondition violation for `signed char`. This test case
was causing `-128` to be passed as a `signed char` to `gcd()`, which is
forbidden.
* libcxx/test/std/containers/sequences/array/assert.iterators.pass.cpp
*
libcxx/test/std/containers/sequences/vector/vector.modifiers/assert.push_back.invalidation.pass.cpp
*
libcxx/test/std/input.output/iostream.format/print.fun/no_file_description.pass.cpp
+ Split some REQUIRES and XFAIL lines. This is a "nice to have" for
MSVC's internal test harness, which is extremely simple and looks for
exact comment matches to skip tests. We can recognize the specific lines
"REQUIRES: has-unix-headers" and "XFAIL: msvc", but it's a headache to
maintain if they're chained with other conditions.
* libcxx/test/support/sized_allocator.h
+ Fix x86 truncation warnings. `std::allocator` takes `std::size_t`, so
we need to `static_cast`.
*
libcxx/test/std/input.output/file.streams/fstreams/ifstream.members/offset_range.pass.cpp
+ Fix x86 truncation warning. `std::min()` is returning
`std::streamoff`, which was being unnecessarily narrowed to
`std::size_t`.
*
libcxx/test/std/algorithms/alg.sorting/alg.merge/inplace_merge_comp.pass.cpp
+ Fix MSVC warning C4127 "conditional expression is constant" for an
always-true branch. This was very recently introduced by #129008 making
`N` constexpr. As it's a local constant just nine lines above, we don't
need to test whether 100 is greater than 0.