GlobalOpt should maintain externally_initialized when splitting aggregates

When GlobalOpt splits an internal, global variable with an aggregate type, it
should propagate the externally_initialized flag to the newly created globals.

This makes the pass safe for our downstream use of this flag, while still
allowing some useful optimisations (such as removing dead parts of the split
aggregate) to be performed.

Differential Revision: http://reviews.llvm.org/D13382

llvm-svn: 252490