Scalar overloads of copy functions

@keryell noted in a [review ](https://github.com/intel/llvm/pull/4686#discussion_r751910150) that it would be useful to have scalar overloads of the queue and handler copy functions. This likely makes sense and applies to SYCL broadly, so tracking here.

Start with the linked review discussion and the SYCL specification sections for queue and handler copy functions. Determine the intended scalar overloads and their scope before updating the specification; done means the supported overloads and their behavior are clearly documented.

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