[Utils] Make -assume-builder/-assume-simplify actually work on Old-PM

clang w/ old-pm currently would simply crash
when -mllvm  -enable-knowledge-retention=true is specified.

Clearly, these two passes had no Old-PM test coverage,
which would have shown the problem - not requiring AssumptionCacheTracker,
but then trying to always get it.

Also, why try to get domtree only if it's cached,
but at the same time marking it as required?