Investigate new LLVM pass manager API

[pitrou]

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

[eliben]

FWIW, the "new" pass manager in LLVM is not yet complete and isn't being used by C++ clients. It's still a WIP