Generate FFI for PassManagerBuilder.

Note that as of LLVM 3.2, there is a bug in LLVM source which will
prevent ffi_gen from working. Manually add stdbool.h to the list of
imported headers and then clean the unrelated definitions it produces.
Add PassManagerBuilder interface.
Merge remote-tracking branch 'whitequark/pmbuilder'
Fix scope error
Add some pass manager builder tests