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

[Sean McBride via cfe-commits]

seanm 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;
      |                             ^
```

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