[analyzer] Base regions may be invalid when layered on symbolic regions.

While RegionStore checks to make sure casts on TypedValueRegions are valid,
it does not do the same for SymbolicRegions, which do not have perfect type
info anyway. Additionally, MemRegion::getAsOffset does not take a
ProgramState, so it can't use dynamic type info to determine a better type
for the regions. (This could also be dangerous if the type of a super-region
changes!)

Account for this by checking that a base object region is valid on top of a
symbolic region, and falling back to "symbolic offset" mode if not.

Fixes PR15345.