[clang][dataflow] Fix handling of `DeclRefExpr`s to `BindingDecl`s.

The invariants around `ReferenceValues` are subtle (arguably, too much so). That
includes that we need to take care not to double wrap them -- in cases where we
wrap a loc in an `ReferenceValue` we need to be sure that the pointee isn't
already a `ReferenceValue`.  `BindingDecl` introduces another situation in which
this can arise. Previously, the code did not properly handle `BindingDecl`,
resulting in double-wrapped values, which broke other invariants (at least, that
struct values have an `AggregateStorageLocation`).

This patch adjusts the interpretation of `DeclRefExpr` to take `BindingDecl`'s
peculiarities into account. It also fixes the two tests which should have caught
this issue but were themselves (subtly) buggy.

Differential Revision: https://reviews.llvm.org/D140897