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

[Sean McBride via cfe-commits]

seanm wrote:

> > This actually compiles, though with a `-Wduplicate-decl-specifier` warning.
> 
> I failed to reproduce this...

I don't have a minimal repro case, I was building [VTK](https://gitlab.kitware.com/vtk/vtk). It's pretty simple to build it yourself if you're familiar with CMake.  I could give you exact command line steps if you need...

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