[PM] Add an enum for describing the desired output strategy, and run
that through the interface rather than a simple bool. This should allow
starting to wire up real output to round-trip IR through opt with the
new pass manager.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@199071 91177308-0d34-0410-b5e6-96231b3b80d8