Trim constexpr from isA to improve Windows clang-cl support.

[ScottTodd]

See #1392, where this code was suggested (#1392 (comment)).

• Without this change, I get build errors when compiling with clang-cl.
• With this change, I can compile FBGEMM and get further along in a full PyTorch on Windows build: [Windows] PyTorch FBGEMM isA compile error needs a patch for clang-cl ROCm/TheRock#598 using clang-cl (working around different errors with MSVC elsewhere in the stack).

Full configure + build logs before/after this change: https://gist.github.com/ScottTodd/31083b69b4b6c5c58cf1579e3dbb3a49 . Generally speaking, constexpr support across compilers is fragile, particularly for more complex cases with composition of multiple functions, classes, etc.

---

I'm specifically using this clang-cl version, built from AMD's LLVM fork at https://github.com/ROCm/llvm-project by https://github.com/ROCm/TheRock:

λ D:\projects\TheRock\build\compiler\amd-llvm\dist\lib\llvm\bin\clang-cl.exe --version
AMD clang version 19.0.0git (https://github.com/ROCm/llvm-project.git 2012c1d23e398aa7fca45ba722fdd97c39f34491)
Target: x86_64-pc-windows-msvc
Thread model: posix
InstalledDir: D:\projects\TheRock\build\compiler\amd-llvm\dist\lib\llvm\bin

FWIW, when I try with an older version from MSVC, I get different errors:

λ clang-cl --version
clang version 18.1.8
Target: x86_64-pc-windows-msvc
Thread model: posix
InstalledDir: C:\Program Files (x86)\Microsoft Visual Studio\2022\BuildTools\VC\Tools\Llvm\x64\bin

[1/4] Building CXX object CMakeFiles\fbgemm_generic.dir\src\spmmUtils.cc.obj
FAILED: CMakeFiles/fbgemm_generic.dir/src/spmmUtils.cc.obj 
C:\PROGRA~2\MICROS~2\2022\BUILDT~1\VC\Tools\Llvm\x64\bin\clang-cl.exe  /nologo -TP  -ID:\projects\FBGEMM\external\cpuinfo\include -ID:\projects\FBGEMM\external\asmjit\src -ID:\projects\FBGEMM\include -ID:\projects\FBGEMM /DWIN32 /D_WINDOWS /EHsc -Xclang -fopenmp /wd4244 /wd4267 /wd4305 /wd4309 /O2 /Ob2 /DNDEBUG -std:c++20 -MD /showIncludes /FoCMakeFiles\fbgemm_generic.dir\src\spmmUtils.cc.obj /FdCMakeFiles\fbgemm_generic.dir\ -c -- D:\projects\FBGEMM\src\spmmUtils.cc
In file included from D:\projects\FBGEMM\src\spmmUtils.cc:10:
In file included from D:\projects\FBGEMM\include\fbgemm/spmmUtils.h:10:
In file included from C:\Program Files (x86)\Microsoft Visual Studio\2022\BuildTools\VC\Tools\MSVC\14.42.34433\include\chrono:25:
C:\Program Files (x86)\Microsoft Visual Studio\2022\BuildTools\VC\Tools\MSVC\14.42.34433\include\format(3205,54): error: capturing a structured binding is not yet supported in OpenMP
 3205 |         return _STD _Widen_and_copy<_CharT>(_Buffer, _End, _STD move(_Out));
      |                                                      ^
C:\Program Files (x86)\Microsoft Visual Studio\2022\BuildTools\VC\Tools\MSVC\14.42.34433\include\format(3189,17): note: '_End' declared here
 3189 |     const auto [_End, _Ec] = _STD to_chars(_Buffer, _STD end(_Buffer), reinterpret_cast<uintptr_t>(_Value), 16);
      |                 ^

(full logs for that: https://gist.github.com/ScottTodd/fc346a1a9f7a6fe1b23f64fa49cb83ff)

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