More updates to front-end design

More updates after meeting (again) with the front-end team:

* Reduce the responsibilities of the front-end proper.  The front-end
  now does only simple transcription of attributes into IR metadata.

* Create a new LLVM IR pass that:
  - Propagates "used aspect" information from types into the containing
    function (moved from front-end responsibility).
  - Propagates "used aspect" information through the static call graph
    (moved from post-link responsibility).
  - Issues warning messages when "used aspects" do not match
    `[[sycl::requires()]]` attributes.

* Remove the proposed static analysis tool.  The new LLVM IR pass can
  provide the same diagnostics, though the source location information
  may not be quite a good.

* Eliminate the diagnostic in post-link if the used aspects in the
  definition of a `SYCL_EXTERNAL` function does not match the used
  aspects in its declaration.  I think this would be difficult without
  changing the LLVM IR linker.

* Add a new section defining terms.

Author: gmlueck