[Driver] Restrict Ofast deprecation help message to Clang (llvm#101682)

The discussion about this in Flang
(https://discourse.llvm.org/t/rfc-deprecate-ofast-in-flang/80243) has
not concluded hence restricting the deprecation only to Clang.

(cherry picked from commit e60ee1f)

Author: kiranchandramohan

clang/include/clang/Driver/Options.td

@@ -932,8 +932,9 @@ def O_flag : Flag<["-"], "O">, Visibility<[ClangOption, CC1Option, FC1Option]>,
   Alias<O>, AliasArgs<["1"]>;
 def Ofast : Joined<["-"], "Ofast">, Group<O_Group>,
   Visibility<[ClangOption, CC1Option, FlangOption]>,
-  HelpText<"Deprecated; use '-O3 -ffast-math' for the same behavior,"
-  " or '-O3' to enable only conforming optimizations">;
+  HelpTextForVariants<[ClangOption, CC1Option],
+                      "Deprecated; use '-O3 -ffast-math' for the same behavior,"
+                      " or '-O3' to enable only conforming optimizations">;
 def P : Flag<["-"], "P">,
   Visibility<[ClangOption, CC1Option, FlangOption, FC1Option]>,
   Group<Preprocessor_Group>,