[AArch64] Using AArch64TargetParser in Clang.

This resubmit r270688 which broke some specific buildbots.That's because
there is incorrect indexing problem in the targetparser,and the problem is
fixed in r276957.

Differential Revision: https://reviews.llvm.org/D21277

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@276958 91177308-0d34-0410-b5e6-96231b3b80d8