[clang] [clang] Reject character devices in #embed for now (PR #135370)

[Aaron Ballman via cfe-commits]

AaronBallman wrote:

> I'm not sure what to say in the release note. Sorry folks, we never really supported these but now we reject to be safe?

I would say something like "the '#embed' directive now diagnoses use of a non-character file (device file) such as `/dev/urandom` as an error. This restriction may be relaxed in the future." or something along those lines.

https://github.com/llvm/llvm-project/pull/135370