[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