[NFC][DirectX] Moving IsValidXYZ Functions to cpp file (#161896)
Originally, DXContainer `isValid...` functions were defined din the
header. That cause a problem, that made this specific file huge, since
every function also includes a `def` file. To fix this problem, this PR
moves the function to the cpp equivalent file.
Closes: [158162](https://github.com/llvm/llvm-project/issues/158162)