[Spec change] Add constraint to vec <<, >> operators

[sycl-issue-bot]

Please review whether KhronosGroup/SYCL-Docs#664 by @gmlueck requires any changes to the CTS.

If changes are required: Open a new PR addressing the changes and link it to this issue.
If no changes are required: Close this issue and proceed with the spec PR.

[gmlueck]

I think there is no need for a CTS change. This change adds a new constraint, so the only possible change would be to remove testing for these argument types. However, the CTS must not be testing these argument types because they do not work in DPC++ and DPC++ passes the CTS.