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

[Andy Kaylor via cfe-commits]

andykaylor wrote:

> 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.

As I just mentioned on my RFC, it turns out the state of backend support is worse than I thought. I started with NVPTX and quickly realized that I wasn't going to be able to create an easy fix that I could just tack on to this PR, because their instruction selection pattern matching code is relying on the global flag without passing the nodes involved to the function that does the check.

I'm going to close this PR and try to start of push for the backends to clean up their handling and respect what the IR says rather than relying on the global setting.

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