ARM64: use the alternate string layout on Apple platforms.
ARM64: use the alternate string layout on Apple platforms.

git-svn-id: 756ef344af921d95d562d9e9f9389127a89a6314@205138 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/libcxx,llvm-mirror/libcxx,llvm-mirror/libcxx,llvm-mirror/libcxx,llvm-mirror/libcxx