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

[Sean McBride via cfe-commits]

seanm wrote:

I've just tried VTK again, and seems there's only one compiler error now:

```diff
-  operator bool() const noexcept { return this->InT >= 0.0 && this->OutT >= 0.0; }
+  operator constexpr bool() const noexcept { return this->InT >= 0.0 && this->OutT >= 0.0; }
```

```
VTK/Filters/ParallelDIY2/vtkProbeLineFilter.cxx:73:12: error: type name does not allow constexpr specifier to be specified
   73 |   operator constexpr bool() const noexcept { return this->InT >= 0.0 && this->OutT >= 0.0; }
      |            ^
```



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