[clang] [clang] Add deprecation warning for `-Ofast` driver option (PR #98736)

[Vlad Serebrennikov via cfe-commits]

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@@ -442,6 +442,9 @@ def warn_drv_deprecated_arg : Warning<
 def warn_drv_deprecated_arg_no_relaxed_template_template_args : Warning<
   "argument '-fno-relaxed-template-template-args' is deprecated">,
   InGroup<DeprecatedNoRelaxedTemplateTemplateArgs>;
+def warn_drv_deprecated_arg_ofast : Warning<
+  "argument '-Ofast' is deprecated; use '-O3', possibly with '-ffast-math'">,
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Endilll wrote:

I think @mizvekov raised a good point, so I tried to take it into account.
@jyknight @AaronBallman What do you think of the new wording (`use '-O3 -ffast math' instead, or just '-O3' if you don't intend to enable non-conforming optimizations`)?

https://github.com/llvm/llvm-project/pull/98736