[clang][dataflow] Update StructValue child when assigning a value

When assigning a value to a storage location of a struct member we
need to also update the value in the corresponding `StructValue`.

This is part of the implementation of the dataflow analysis framework.
See "[RFC] A dataflow analysis framework for Clang AST" on cfe-dev.

Reviewed-by: ymandel, xazax.hun

Differential Revision: https://reviews.llvm.org/D120414