Investigate new LLVM pass manager API

Currently, we go through the LLVM C API which instantiates "legacy" pass managers, e.g.
http://llvm.org/docs/doxygen/html/classllvm_1_1legacy_1_1FunctionPassManager.html

We may want to invoke a C++ API directly instead, in order to get new pass managers, as in
http://llvm.org/docs/doxygen/html/classllvm_1_1FunctionPassManager.html

The changes seem to be covered in:
- https://groups.google.com/forum/#!topic/llvm-dev/_4oXsK-hI5E
- http://lists.cs.uiuc.edu/pipermail/llvmdev/2012-July/051638.html

Start by reviewing the current use of the LLVM C API and the legacy::FunctionPassManager, then compare it with the LLVM FunctionPassManager C++ API and the linked LLVM discussions. Done means determining whether direct C++ access is feasible and producing a scoped implementation plan.

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