Switch over to using object_getClass to get the class of an object.  Previously we were
directly accessing the isa pointer of a class object to get its meta-class, but the isa
pointers are not simple pointers on arm64, so this would cause the stepping to fail.
object_getClass does whatever magic needs doing in this case.

<rdar://problem/17239690>


git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@211289 91177308-0d34-0410-b5e6-96231b3b80d8