swift234

[irgen] Refactor out the pass pipeline build code in IRGen so that llvm-swift-opt can use it.

llvm-swift-opt is a swift only version of opt that will be used to remove
swift's dependency on dynamically injecting passes into opt and to allow for
IRGen's LLVM pipeline to be simulated exactly by reusing the IRGen pass pipeline
building code.

The reason to remove the dynamic code is that it seems that this is code that
will be in flux vis-a-vis the transition to the new pass manager in LLVM, so it
makes sense to make this change now due to the benefit provided by exactly
simulating IRGen.

Swift SVN r32804