[analyzer] Fix uninitialized base class with initializer list when ctor is not...

[analyzer] Fix uninitialized base class with initializer list when ctor is not declared in the base class

Fixes #70464

When ctor is not declared in the base class, initializing the base class
with the initializer list will not trigger a proper assignment of the
base region, as a CXXConstructExpr doing that is not available in the
AST.

This patch checks whether the init expr is an InitListExpr under a base
initializer, and adds a binding if so.