ARM64: use the alternate string layout on Apple platforms.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@205138 91177308-0d34-0410-b5e6-96231b3b80d8
ARM64: compare RTTI names as strings

ARM64 generates RTTI with hidden visibility, which means that typeinfo
must be compared char-by-char since it's not guaranteed to be uniqued
across the whole program.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@205139 91177308-0d34-0410-b5e6-96231b3b80d8