[clang] Honor pragmas with -ffp-contract=fast, depecate fast-honor-pragmas (PR #105746)

[Eli Friedman via cfe-commits]

https://github.com/efriedma-quic commented:

Is the backend actually ready for this?  Looking quickly at the backend, it looks like target-independent code is fine, but some of the target-specific code doesn't respect the "contract" flag on instructions.

https://github.com/llvm/llvm-project/pull/105746