[clang][dataflow] Assign aggregate storage locations to union stmts

This patch ensures that the dataflow analysis framework does not crash
when it encounters access to members of union types.

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: xazax.hun

Differential Revision: https://reviews.llvm.org/D118226