One exception here is LoopInfo which must forward-declare it (because
the typedef is in LoopPassManager.h which depends on LoopInfo).

Also, some includes for LoopPassManager.h were needed since that file
provides the typedef.

Besides a general consistently benefit, the extra layer of indirection
allows the mechanical part of https://reviews.llvm.org/D23256 that
requires touching every transformation and analysis to be factored out
cleanly.

Thanks to David for the suggestion.

llvm-svn: 278079