Copy missing member in DataLayout copy ctor.

Test case is missing due to it not being reachable through the current tools
but out of tree code such as the sample at
http://llvm.org/docs/tutorial/LangImpl4.html

Patch by Peng Cheng <gm4cheng@gmail.com>

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@176213 91177308-0d34-0410-b5e6-96231b3b80d8