[clang] [NFC][Matrix][Clang][HLSL] Move MaxMatrixDimension to a LangOpt (PR #163307)

[Chris B via cfe-commits]

https://github.com/llvm-beanz requested changes to this pull request.

This change isn't NFC. You're not just moving a set value out of a header, you're also changing how it applies to the underlying `matrix_type` attribute. That warrants tests.

https://github.com/llvm/llvm-project/pull/163307