[clang][auto-init] Deprecate...

[clang][auto-init] Deprecate -enable-trivial-auto-var-init-zero-knowing-it-will-be-removed-from-clang

GCC 12 has been released and contains unconditional support for
-ftrivial-auto-var-init=zero:
https://gcc.gnu.org/onlinedocs/gcc/Optimize-Options.html#index-ftrivial-auto-var-init

Maintain compatibility with GCC, and remove the -enable flag for "zero"
mode. The flag is left to generate an "unused" warning, though, to not
break all the existing users. The flag will be fully removed in Clang 17.

Link: https://github.com/llvm/llvm-project/issues/44842

Reviewed By: nickdesaulniers, MaskRay, srhines, xbolva00

Differential Revision: https://reviews.llvm.org/D125142