[clang][dataflow] Include fields initialized in an `InitListExpr` in `getModeledFields()`.

Previously, we were including these fields only in the specific instance that was initialized by the `InitListExpr`, but not in other instances of the same type. This is inconsistent and error-prone.

Depends On D154952

Reviewed By: xazax.hun, gribozavr2

Differential Revision: https://reviews.llvm.org/D154961