[lib/LTO] Factor out logic for running passes.
[lib/LTO] Factor out logic for running passes.

This is in preparation for adding an option
to run a custom pipeline with the new PM. It's
currently used in lld.


git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@280258 91177308-0d34-0410-b5e6-96231b3b80d8

apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm