Add a separate llvm.global_ctors entry for linkonce_odr data initializers

Summary:
These typically come from static data members of class template
specializations.  This accomplishes two things:

1. May expose GlobalOpt optimizations for Itanium C++ ABI code.
2. Works toward fixing double initialization in the Microsoft C++ ABI.

CC: cfe-commits

Differential Revision: http://llvm-reviews.chandlerc.com/D1475