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

[Zahira Ammarguellat via cfe-commits]

zahiraam wrote:

> double foo();
> double d = 2.0 * foo();

That's correct. Only for division and only `-fcomplex-arithmetic=promoted` and when the higher precision type of a target is the same than the current precision type.

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