Change references to 'const ProgramState *' to typedef 'ProgramStateRef'.

Change references to 'const ProgramState *' to typedef 'ProgramStateRef'.

At this point this is largely cosmetic, but it opens the door to replace
ProgramStateRef with a smart pointer that more eagerly acts in the role
of reclaiming unused ProgramState objects.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@149081 91177308-0d34-0410-b5e6-96231b3b80d8