[clang] Do not warn unused -enable-trivial-auto-var-init-zero-knowing-it-will-be-removed-from-clang

[kongyi]

Summary

Android enables zero initialisation globally by default, but also allows subprojects to override with different option. Clang complains the above flag being unused in this case.

Instead of adding a 75 char long -no-* flag, don't warn unused argument for this flag.

[glider]

I don't have a strong opinion on this. On one hand, passing the correct flags should be something best taken care of by the build system.
On the other hand, letting this flag proliferate is a good thing to do, so let's make the users' life a bit easier.

clang/include/clang/Driver/Options.td

HelpText<"Enable stack protectors for some functions vulnerable to stack smashing. "
            "address. The reference stack guard value is stored in a global variable.">;
 def ftrivial_auto_var_init : Joined<["-"], "ftrivial-auto-var-init=">, Group<f_Group>,
   Flags<[CC1Option, CoreOption]>, HelpText<"Initialize trivial automatic stack variables: uninitialized (default)"
   " | pattern">, Values<"uninitialized,zero,pattern">,
   NormalizedValuesScope<"LangOptions::TrivialAutoVarInitKind">,
   NormalizedValues<["Uninitialized", "Zero", "Pattern"]>,
   MarshallingInfoEnum<LangOpts<"TrivialAutoVarInit">, "Uninitialized">;
 def enable_trivial_var_init_zero : Flag<["-"], "enable-trivial-auto-var-init-zero-knowing-it-will-be-removed-from-clang">,
-  Flags<[CC1Option, CoreOption]>,
+  Flags<[CC1Option, CoreOption, NoArgumentUnused]>,
   HelpText<"Trivial automatic variable initialization to zero is only here for benchmarks, it'll eventually be removed, and I'm OK with that because I'm only using it to benchmark">;
 def ftrivial_auto_var_init_stop_after : Joined<["-"], "ftrivial-auto-var-init-stop-after=">, Group<f_Group>,
   Flags<[CC1Option, CoreOption]>, HelpText<"Stop initializing trivial automatic stack variables after the specified number of instances">,
   MarshallingInfoInt<LangOpts<"TrivialAutoVarInitStopAfter">>;
 def fstandalone_debug : Flag<["-"], "fstandalone-debug">, Group<f_Group>, Flags<[CoreOption]>,
   HelpText<"Emit full debug info for all types used by the program">;
 def fno_standalone_debug : Flag<["-"], "fno-standalone-debug">, Group<f_Group>, Flags<[CoreOption]>,
   HelpText<"Limit debug information produced to reduce size of debug binary">;