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

[Sean McBride via cfe-commits]

seanm wrote:

Another interesting case:

```c++
static int testToWindowsExtendedPath()
{
#ifdef _WIN32
  // lots of real, non constant work
  return ret;
#else
  return 0;
#endif
}
```

For some platforms this can be constexpr; for other platforms (Windows), it cannot. It was transformed to `constexpr` since I'm on Mac. Not sure what you can do there...

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