Unions reinitialized after assignment into field.

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 #21232, #54499, #54986).

src/test/ui/borrowck/borrowck-union-move-assign.nll.stderr

@@ -8,28 +8,6 @@ LL |             let a = u.a; //~ ERROR use of moved value: `u.a`
    |
    = note: move occurs because `u` has type `U`, which does not implement the `Copy` trait
 
-error[E0382]: use of moved value: `u`
-  --> $DIR/borrowck-union-move-assign.rs:33:21
-   |
-LL |             let a = u.a;
-   |                     --- value moved here
-LL |             u.a = A;
-LL |             let a = u.a; // OK
-   |                     ^^^ value used here after move
-   |
-   = note: move occurs because `u` has type `U`, which does not implement the `Copy` trait
-
-error[E0382]: use of moved value: `u`
-  --> $DIR/borrowck-union-move-assign.rs:39:21
-   |
-LL |             let a = u.a;
-   |                     --- value moved here
-LL |             u.b = B;
-LL |             let a = u.a; // OK
-   |                     ^^^ value used here after move
-   |
-   = note: move occurs because `u` has type `U`, which does not implement the `Copy` trait
-
-error: aborting due to 3 previous errors
+error: aborting due to previous error
 
 For more information about this error, try `rustc --explain E0382`.

src/test/ui/borrowck/reassignment_immutable_fields_overlapping.nll.stderr

@@ -4,6 +4,16 @@ error[E0381]: assign to part of possibly uninitialized variable: `x`
 LL |     x.a = 1;  //~ ERROR
    |     ^^^^^^^ use of possibly uninitialized `x`
 
-error: aborting due to previous error
+error[E0594]: cannot assign to `x.b`, as `x` is not declared as mutable
+  --> $DIR/reassignment_immutable_fields_overlapping.rs:23:5
+   |
+LL |     let x: Foo;
+   |         - help: consider changing this to be mutable: `mut x`
+LL |     x.a = 1;  //~ ERROR
+LL |     x.b = 22; //~ ERROR
+   |     ^^^^^^^^ cannot assign
 
-For more information about this error, try `rustc --explain E0381`.
+error: aborting due to 2 previous errors
+
+Some errors occurred: E0381, E0594.
+For more information about an error, try `rustc --explain E0381`.