[analyzer] Allow padding checker to traverse simple class hierarchies

The existing padding checker skips classes that have any base classes. 
This patch allows the checker to traverse very simple cases: 
classes that have no fields and have exactly one base class. 
This is important mostly in the case of array declarations.

Patch by Max Bernstein!

Test plan: make check-all

Differential revision: https://reviews.llvm.org/D53206

llvm-svn: 345558