Change the BugDriver to store the current module with std::unique_ptr.

While there, change a bunch of helper functions to take references to
avoid adding calls to get().

This should conclude the bugpoint yak shaving.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@325177 91177308-0d34-0410-b5e6-96231b3b80d8
8 files changed