ARM64: parametrise IVar offset type (long on ARM64, int elsewhere).

This is part of the ARM64 patch, but can only be tested properly when
the full codegen gets committed.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@205098 91177308-0d34-0410-b5e6-96231b3b80d8