ImathTest does not build with Apple Clang C++ 20

Apple Clang does not support `std::bit_cast()` which is used in src/ImathTest/testFun.cpp (https://en.cppreference.com/w/cpp/compiler_support), using Imath 3.1.6.
The problem could be resolved by using the already defined polyfill in the same file for Apple Clang.

So line 23 in src/ImathTest/testFun.cpp:
```c++
#if __cplusplus < 202002L
```

could become something like:
```c++
#if __cplusplus < 202002L || (__clang__ && __APPLE__)
```

Also line 11 in the same file could be changed to:
```c++
#if __cplusplus >= 202002L && !(__clang__ && __APPLE__)
```
since including is then no longer required.

Start with src/ImathTest/testFun.cpp and inspect the existing polyfill and the two preprocessor guards described in the issue. Build ImathTest with Apple Clang in C++20 mode to confirm the failure, then verify that the test target builds successfully without requiring .

Written by the indexing model from the issue text.