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

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wjristow wrote:

Over in #162549, I proposed what is essentially a PlayStation-specific version of the suggestion of this PR.  In discussion there, people indicated that revisiting this PR may be a better approach (rather than doing a PlayStation-specific fix), because things have improved in various backends since this PR was first proposed.

I don't have a good handle on which backends would run into trouble, but in that [other PR](https://github.com/llvm/llvm-project/pull/162549#issuecomment-3383666444), @efriedma-quic suggested:

> We could blacklist specific backends which still have issues.

That sounds like a good approach to me.  (Alternatively, we could do the PlayStation-specific change of #162549, and others that want the behavior of the pragmas always being honored could "opt in" to using the approach that PlayStation is using.  Essentially _evolving toward the approach_ suggested in this PR, rather than forcing it on all backends at once.)

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