[SYCL][FE]Diagnostic message improvement for negative arguments values of work_group_size attributes

[smanna12]

Possible alternative: we could also reuse the existing diagnostic text about implicit conversion, but put it under the new attribute grouping:

def warn_attribute_requires_non_negative_integer_argument : Warning<warn_impcast_integer_sign.Text>, InGroup<AcceptedAttributes>;

One oddity from this would be that users who pass -Wno-sign-compare will still get what looks like a sign comparison warning, but that's the same behavior as with the current patch anyway.

Originally posted by @AaronBallman in #2906 (comment)

[smanna12]

Fixed by #2988