This allows MemRegion and MemRegionManager to avoid asking over and over
again whether an class is a virtual base or a non-virtual base.

Minor optimization/cleanup; no functionality change.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@175716 91177308-0d34-0410-b5e6-96231b3b80d8