This commit makes two changes:

First, it updates the dataflow builder to add an init for the place
containing a union if there is an assignment into the field of
that union.

Second, it stops a "use of uninitialized" error occuring when there is an
assignment into the field of an uninitialized union that was previously
initialized. Making this assignment would re-initialize the union, as
tested in `src/test/ui/borrowck/borrowck-union-move-assign.nll.stderr`.
The check for previous initialization ensures that we do not start
supporting partial initialization yet (cc rust-lang#21232, rust-lang#54499, rust-lang#54986).

This commit adds a run-pass test for the subset of
`src/test/ui/borrowck/borrowck-union-move-assign.rs` that is intended to
pass as the union is reinitialized.

It is necessary to detect whether we are making the first
assignment into a union. This is checked by looking at the moves and
checking if there are any from locations earlier in the control flow
graph.

This commit improves the detection of this by switching from a naive
method that compared only the statement and basic block indices with
a more robust method that looks at the predecessors of a location.