[clang][dataflow] Fix Record initialization with InitListExpr and inheritances

Usually RecordValues for record objects (e.g. struct) are initialized with
`Environment::createValue()` which internally calls `getObjectFields()` to
collects all fields from the current and base classes, and then filter them
with `ModeledValues` via `DACtx::getModeledFields()` so that the fields that
are actually referenced are modeled.

The consistent set of fields should be initialized when a record is initialized
with an initializer list (InitListExpr), however the existing code's behavior
was different.

Before this patch:
* When a struct is initialized with InitListExpr, its fields are
  initialized based on what is returned by `getFieldsForInitListExpr()`, which
  only collects the direct fields in the current class, but not from the base
  classes. Moreover, if the base classes have their own InitListExpr, values
  that are initialized by their InitListExpr's weren't merged into the
  child objects.

After this patch:
* When a struct is initialized with InitListExpr, it collects and merges the
  fields in the base classes that were initialized by their InitListExpr's.
  The code also asserts that the consistent set of fields are initialized
  with the ModeledFields.

Reviewed By: mboehme

Differential Revision: https://reviews.llvm.org/D159284