[analyzer] Allow padding checker to traverse simple class hierarchies

[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

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@345558 91177308-0d34-0410-b5e6-96231b3b80d8