Construct exhaustiveness branches with condition, not branch, variable

Previously we would construct the shapes of unions used in the pattern
tree for exhaustiveness checking using the type of the branch patterns,
rather than the type of the condition variable. Clearly we want to
always use the condition variable, otherwise some branches will be
seen as exhaustive, when they are not!

To do this, we now index into the condition variable while refying the
patterns to build the tree for exhaustiveness checking.

Closes #4068

crates/compiler/solve/src/solve.rs

@@ -1480,7 +1480,7 @@ fn solve(
                         errors,
                         exhaustive,
                         redundancies,
-                    } = check(subs, sketched_rows, context);
+                    } = check(subs, real_var, sketched_rows, context);
 
                     // Store information about whether the "when" is exhaustive, and
                     // which (if any) of its branches are redundant. Codegen may use