[PM/AA] Extract a minimal interface for CFLAA to its own header file.

I've used forward declarations and reorderd the source code some to make
this reasonably clean and keep as much of the code as possible in the
source file, including all the stratified set details. Just the basic AA
interface and the create function are in the header file, and the header
file is now included into the relevant locations.

llvm-svn: 245009