[clang] [clang][SYCL] Add sycl_external attribute and restrict emitting device code (PR #140282)

[Erich Keane via cfe-commits]

https://github.com/erichkeane commented:

Ah shoot, apparently I forgot to click 'submit' when I reviewed this last week.  Sorry about that.  Here are my pending comments.

https://github.com/llvm/llvm-project/pull/140282