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

[Aaron Ballman via cfe-commits]

https://github.com/AaronBallman commented:

We're currently only tracking the precision loss for complex division, but doesn't it also apply to any use of a floating-point type? e.g.,
```
double foo();
double d = 2.0 * foo();
```
would still not use the higher precision, right? The only difference is that complex division can see an observable algorithmic change, so we want to issue the "which may result in observable differences" part of the diagnostic for that case, but otherwise we would drop that bit from the diagnostic.

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