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

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.

Differential Revision: https://reviews.llvm.org/D108278

Author: kongy

clang/include/clang/Driver/Options.td

@@ -2561,7 +2561,7 @@ def ftrivial_auto_var_init : Joined<["-"], "ftrivial-auto-var-init=">, Group<f_G
   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">,