[clang] [SYCL] Error on C inputs when compiling with -fsycl (PR #200318)

[Aaron Ballman via cfe-commits]

https://github.com/AaronBallman approved this pull request.

Precommit CI failure is unrelated, the changes LGTM but please let one of the SYCL folks sign-off too before landing.

https://github.com/llvm/llvm-project/pull/200318