[clang] [NFC] Move warning from CodeGen to Sema. (PR #107397)

[Aaron Ballman via cfe-commits]

https://github.com/AaronBallman commented:

Double-checking: we want to diagnose when higher precision is requested but cannot be honored and failing to honor the request leads to an observable change in behavior and the only time that should happen currently is with complex division, so that's why it's the only situation we diagnose. Is that accurate?

CC @jcranmer-intel @andykaylor to make sure I'm not missing anything

https://github.com/llvm/llvm-project/pull/107397