Re-add DataLayout pointer size convenience functions.

These were reverted in r167222 along with the rest
of the last different address space pointer size attempt.
These will be used in later commits.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@187223 91177308-0d34-0410-b5e6-96231b3b80d8