Add a public accessor for GlobalCtors in CodeGenModule

Summary:
In a JIT context it is useful to be able to access the GlobalCtors
and especially clear them once they have been emitted and called.
This adds a public method to be able to access the list.

Subscribers: yaron.keren, cfe-commits

Differential Revision: http://reviews.llvm.org/D6790

llvm-svn: 224982