[wgsl-in] Record identifier definitions as TypedExpression values.

Both a `var` binding and a `let` binding of a pointer to a variable create
entries in the `lookup_ident` tables whose expression is a `LocalVariable`.
However, the `let` should be a pointer, whereas the `var` should be a reference.
This patch changes the tables to hold `TypedExpression` values, thus preserving
the distinction.