[clang][SYCL] Add sycl_external attribute and restrict emitting device code

[schittir]

This patch is part of the upstreaming effort for supporting SYCL language front end.
It makes the following changes:

1. Adds sycl_external attribute for functions
2. Adds checks to avoid emitting code when sycl_external and sycl_kernel_entry_point attributes are not enabled
3. Fixes test failures caused by the above changes

[tahonermann]

Thanks for working on this @schittir! I completed an initial pass of all of the code, but still need to look more closely at the documentation updates.

[schittir]

Thanks for working on this @schittir! I completed an initial pass of all of the code, but still need to look more closely at the documentation updates.

Thank you for the initial pass review, Tom!

[github-actions]

✅ With the latest revision this PR passed the C/C++ code formatter.

[tahonermann]

Thanks @schittir, here are my initial review comments. I expect to review the newly added tests more closely once these comments are all addressed.

[tahonermann]

Thanks for addressing most of my earlier comments. I took another quick look today and added a couple more comments.

[tahonermann]

Additional comments for diagnostics and tests.

[Fznamznon]

Oops new tests are failing.