[analyzer] Don't initialize virtual base classes more than once.

In order to make sure virtual base classes are always initialized once,
the AST contains initializers for the base class in /all/ of its
descendents, not just the immediate descendents. However, at runtime,
the most-derived object is responsible for initializing all the virtual
base classes; all the other initializers will be ignored.

The analyzer now checks to see if it's being called from another base
constructor, and if so does not perform virtual base initialization.

<rdar://problem/14236851>

llvm-svn: 184814