Reapply most of r82939, but add a guard that FieldRegions and friends

are only specially treated by RegionStore::InvalidateRegion() when
their super region is also invalidated.  When this isn't the case,
conjure a new symbol for a FieldRegion.  Thanks to Zhongxing Xu and
Daniel Dunbar for pointing out this issue.

llvm-svn: 83043