Extend Module::getOrInsertGlobal to control the construction of the

GlobalVariable

Summary:
Extend Module::getOrInsertGlobal to accept a callback for creating a new
GlobalVariable if necessary instead of calling the GV constructor
directly using default arguments. Additionally overload
getOrInsertGlobal for the previous default behavior.

Reviewers: chandlerc

Subscribers: hiraditya, llvm-commits, bollu

Differential Revision: https://reviews.llvm.org/D56130

llvm-svn: 350219