[clang] [clang] Set correct FPOptions if attribute 'optnone' presents (PR #85605)

[Serge Pavlov via cfe-commits]

spavloff wrote:

Hi @dyung.

The observed difference is due to the FP contraction turned off if optnone is specified. In O0 this optimization is still applied. As a result, the function with optnone contains separate fadd and fmul, while without this attribute the function contains combined operatin fmuladd.

This optimization is turned off in `FPOption.setDisallowOptimizations` (introduced by this patch) by the call `setDisallowFPContract()`. If it is removed, I think, the output would be identical.

https://github.com/llvm/llvm-project/pull/85605