[clang] [NFC] Refactor fast-math handling for clang driver (PR #81173)

[Fangrui Song via cfe-commits]

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@@ -3061,22 +3080,7 @@ static void RenderFloatingPointOptions(const ToolChain &TC, const Driver &D,
         continue;
       [[fallthrough]];
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MaskRay wrote:

Should fallthrough still be used?

https://github.com/llvm/llvm-project/pull/81173