Compile Error When Testing with MSVC Using /std:c++latest

Hi, I work on Microsoft Visual C++ testing, where we regularly build popular open-source projects, including Boost, with development builds of our compiler and libraries with some particular CL options such as `/std:c++latest`.

Feel free to close this issue if there is no plan of supporting `/std:c++latest` in the future. This change in `/std:c++latest` will be released soon in next VS update.
``
While building and testing Boost from source using `/std:c++latest`, I encountered the following error in the `phoenix` test:
```c++
libs\phoenix\test\statement\if_tests.cpp(33): error C2676: binary '[': 'const boost::phoenix::if_gen>' does not define this operator or a conversion to a type acceptable to the predefined operator
```

This error was encountered in:
[test\statement\if_tests.cpp](https://github.com/boostorg/phoenix/blob/develop/test/statement/loops_tests.cpp) (Line 33, 48, 55)
[test\statement\loops_tests.cpp](https://github.com/boostorg/phoenix/blob/develop/test/statement/loops_tests.cpp) (Line 40, 56, 74)
[test\statement\switch_tests.cpp](https://github.com/boostorg/phoenix/blob/develop/test/statement/switch_tests.cpp) (Line 59, 72)
[test\statement\exceptions.cpp](https://github.com/boostorg/phoenix/blob/develop/test/statement/exceptions.cpp) (Line 128, 145, 174)
[test\bll_compatibility\algorithm_test.cpp](https://github.com/boostorg/phoenix/blob/develop/test/bll_compatibility/algorithm_test.cpp) (Line 54)

In C++23, the definition of a subscript-expression was changed so that it now takes an expression-list so now a top-level ',' is a separator. **The fix is to wrap the expression in parentheses**. For example in loops_test.cpp:
```c++
while_(arg1--)
[
cout << arg1 << ", ",
++ref(x)
],
```

should be changed to:
```c++
while_(arg1--)
[
(cout << arg1 << ", ", // added opening parenthesis
++ref(x)) // added closing parenthesis
],
```

Test log:
[boost_phoenix_test_log.txt.txt](https://github.com/user-attachments/files/19701209/boost_phoenix_test_log.txt.txt)

Start with the listed Phoenix tests, especially test/statement/loops_tests.cpp, and reproduce the failure under MSVC with /std:c++latest. Update the comma expressions identified in if_tests.cpp, loops_tests.cpp, switch_tests.cpp, exceptions.cpp, and bll_compatibility/algorithm_test.cpp as needed. Done means the affected Phoenix tests compile and pass with that compiler mode.

Written by the indexing model from the issue text.