[clang] Clean up denormal handling with -ffp-model, -ffast-math, etc. (PR #89477)

[Fangrui Song via cfe-commits]

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@@ -64,7 +64,8 @@
 
 // RUN: %clang -### -ffp-model=strict -fdenormal-fp-math=preserve-sign,preserve-sign -c %s 2>&1 \
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MaskRay wrote:

You can use `-### -Werror` to ensure that there is no warning.

(In the absence of a `--target=`, you probably want to check a few different targets that the warning is printed regardless of the default target triple.)

https://github.com/llvm/llvm-project/pull/89477