[libc++] [test] Fix MSVC warnings (#93257)

Found while running libc++'s tests with MSVC's STL.

* Avoid MSVC warning C5101: use of preprocessor directive in
function-like macro argument list is u

[libc++] [test] Fix MSVC warnings (#93257)

Found while running libc++'s tests with MSVC's STL.

* Avoid MSVC warning C5101: use of preprocessor directive in
function-like macro argument list is undefined behavior.
+ We can easily make this portable by extracting `const bool is_newlib`.
+ Followup to #73440.
+ See #73598.
+ See #73836.
* Avoid MSVC warning C4267: 'return': conversion from 'size_t' to 'int',
possible loss of data.
+ This warning is valid, but harmless for the test, so
`static_cast<int>` will avoid it.
* Avoid MSVC warning C4146: unary minus operator applied to unsigned
type, result still unsigned.
+ This warning is also valid (the scenario is sometimes intentional, but
surprising enough that it's worth warning about). This is a C++17 test,
so we can easily avoid it by testing `is_signed_v` at compile-time
before testing `m < 0` and `n < 0` at run-time.
* Silence MSVC warning C4310: cast truncates constant value.
+ These warnings are being emitted by `T(255)`. Disabling the warning is
simpler than attempting to restructure the code.
+ Followup to #79791.
* MSVC no longer emits warning C4521: multiple copy constructors
specified.
+ This warning was removed from the compiler, since at least 2021-12-09.

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[libc++][span] P2447R4: `std::span` over an initializer list (#78157)

Implements: https://wg21.link/P2447R6
- https://eel.is/c++draft/span.syn
- https://eel.is/c++draft/span.overview
- https://ee

[libc++][span] P2447R4: `std::span` over an initializer list (#78157)

Implements: https://wg21.link/P2447R6
- https://eel.is/c++draft/span.syn
- https://eel.is/c++draft/span.overview
- https://eel.is/c++draft/span.cons
- https://eel.is/c++draft/diff

---------

Co-authored-by: Zingam <zingam@outlook.com>

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[libc++][NFC] Move span tests under views.span