[libc++][math] Fix acceptance of convertible types in `std::isnan()` and `std::isinf()` (#98952)

Following up on https://github.com/llvm/llvm-project/pull/98841.

Changes:
- Properly test convertible types for `std::isnan()` and `std::inf()`
- Tighten conditional in `cmath.pass.cpp` (Find insights on `_LIBCPP_PREFERRED_OVERLOAD` below)
- Tighten preprocessor guard in `traits.h`

Insights into why `_LIBCPP_PREFERRED_OVERLOAD` is needed:

(i) When libc++ is layered on top of glibc on Linux, glibc's `math.h` is
    included. When compiling with `-std=c++03`, this header brings the
    function declaration of `isinf(double)` [1] and `isnan(double)` [2]
    into scope. This differs from the C99 Standard as only the macros
    `#define isnan(arg)` and `#define isinf(arg)` are expected.

    Therefore, libc++ needs to respect the presense of the `double` overload
    and cannot redefine it as it will conflict with the declaration already
    in scope. For `-std=c++11` and beyond this issue is fixed, as glibc
    guards both the `isinf` and `isnan` by preprocessor macros.

(ii) When libc++ is layered on top of Bionic's libc, `math.h` exposes a
     function prototype for `isinf(double)` with return type `int`. This
     function prototype in Bionic's libc is not guarded by any preprocessor
     macros [3].

`_LIBCPP_PREFERRED_OVERLOAD` specifies that a given overload is a better match
than an otherwise equally good function declaration. This is implemented in
modern versions of Clang via `__attribute__((__enable_if__))`, and not elsewhere.
See [4] for details. We use `_LIBCPP_PREFERRED_OVERLOAD` to define overloads in
the global namespace that displace the overloads provided by the C
libraries mentioned above.

[1]: https://github.com/bminor/glibc/blob/fe9408087583fd7a6f61bb0dbcf2fd4e83186afa/math/bits/mathcalls.h#L185-L194
[2]: https://github.com/bminor/glibc/blob/fe9408087583fd7a6f61bb0dbcf2fd4e83186afa/math/bits/mathcalls.h#L222-L231
[3]: https://cs.android.com/android/platform/superproject/+/master:bionic/libc/include/math.h;l=322-323;drc=master?hl=fr-BE%22https:%2F%2Fsupport.google.com%2Fmerchants%2Fanswer%2F188494%5C%22%22https:%2F%2Fsupport.google.com%2Fmerchants%2Fanswer%2F188494%5C%22
[4]: https://github.com/llvm/llvm-project/commit/5fd17ab1b093f6b59aabb27f6c2c2278e65c2707

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