[analyzer] Fix modeling of constructors

This diff fixes analyzer's crash (triggered assert) on the newly added test case.
The assert being discussed is assert(!B.lookup(R, BindingKey::Direct))
in lib/StaticAnalyzer/Core/RegionStore.cpp, however the root cause is different.
For classes with empty bases the offsets might be tricky.
For example, let's assume we have
 struct S: NonEmptyBase, EmptyBase {
     ...
 };
In this case Clang applies empty base class optimization and 
the offset of EmptyBase will be 0, it can be verified via
clang -cc1 -x c++ -v -fdump-record-layouts main.cpp -emit-llvm -o /dev/null.
When the analyzer tries to perform zero initialization of EmptyBase
it will hit the assert because that region
has already been "written" by the constructor of NonEmptyBase.

Test plan:
make check-all

Differential revision: https://reviews.llvm.org/D36851

llvm-svn: 311182