Don't use native atomics on ivars whose size is not a power of two,

Don't use native atomics on ivars whose size is not a power of two,
even on architectures that support unaligned access (which is the
only way this is otherwise legal, given that ivars apparently do
not honor alignment attributes).

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@139590 91177308-0d34-0410-b5e6-96231b3b80d8