[SYCL] Split diagnostic and AST parts for sema tests.

[smanna12]

Ok. Thanks for explaining. Even if you don't do it in this patch, we need to start splitting diagnostic tests and AST tests. The tests are getting hard to follow because there is so much happening

@elizabethandrews, i agree, I will do this in a separate patch. Thanks

Originally posted by @smanna12 in #4084 (comment)

[github-actions]

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