[clang][dataflow] Fix bug in handling of reference-typed fields.

This patch fixes a subtle bug in how we create lvalues to reference-typed
fields. In the rare case that the field is umodeled because of the depth limit
on field modeling, the lvalue created can be malformed. This patch prevents that
and adds some related assertions to other code dealing with lvalues for
references.

Differential Revision: https://reviews.llvm.org/D142468