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

[Fangrui Song via cfe-commits]

https://github.com/MaskRay commented:

Thanks for doing this! I support this direction as mentioned in #100453

(I'm starting a 3-week vacation this Friday and will have limited availability. Happy for other reviewers to approve.)

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