[clang-tools-extra] [clang-tidy] add modernize-use-constexpr check (PR #146553)

[Julian Schmidt via cfe-commits]

5chmidti wrote:

> Another compiler error after transformation:
> 
> It made this constexpr:
> 
> ```c++
> 	PUGI_IMPL_FN constexpr bool is_nan(double value)
> 	{
> 	#if defined(PUGI_IMPL_MSVC_CRT_VERSION) || defined(__BORLANDC__)
> 		return !!_isnan(value);
> 	#elif defined(fpclassify) && defined(FP_NAN)
> 		return fpclassify(value) == FP_NAN;
> 	#else
> 		// fallback
> 		const volatile double v = value;
> 		return v != v;
> 	#endif
> 	}
> ```
> 
> But then:
> 
> ```
> /Users/sean/external/VTK/ThirdParty/pugixml/vtkpugixml/src/pugixml.cpp:8413:30: error: constexpr function never produces a constant expression [-Winvalid-constexpr]
>  8413 |         PUGI_IMPL_FN constexpr bool is_nan(double value)
>       |                                     ^~~~~~
> /Users/sean/external/VTK/ThirdParty/pugixml/vtkpugixml/src/pugixml.cpp:8422:15: note: read of volatile-qualified type 'const volatile double' is not allowed in a constant expression
>  8422 |                 return v != v;
>       |                             ^
> ```

I'll take a look

https://github.com/llvm/llvm-project/pull/146553