[CSClosure] Avoid function conversion when closure result type is optional Void

In cases like:

```
func test<T>(_: () -> T?) -> [T] {
   ...
}

test {
  if ... {
     return
  }

  ...
}
```

Contextual return type would be first attempted as optional and
then unwrapped if the first attempt did not succeed. To avoid having
to solve the closure twice (which results in an implicit function conversion),
let's add an implicit empty tuple (`()`) to the `return` statement
and allow it be to injected into optional as many times as context
requires.

Author: xedin

lib/Sema/CSClosure.cpp

@@ -1204,8 +1204,35 @@ class ClosureConstraintApplication
   }
 
   ASTNode visitReturnStmt(ReturnStmt *returnStmt) {
-    if (!returnStmt->hasResult())
-      return returnStmt;
+    if (!returnStmt->hasResult()) {
+      // It's possible to infer e.g. `Void?` for cases where
+      // `return` doesn't have an expression. If contextual
+      // type is `Void` wrapped into N optional types, let's
+      // add an implicit `()` expression and let it be injected
+      // into optional required number of times.
+      auto &ctx = closure->getASTContext();
+
+      // If contextual is not optional, there is nothing to do here.
+      if (resultType->isVoid())
+        return returnStmt;
+
+      assert(resultType->getOptionalObjectType() &&
+             resultType->lookThroughAllOptionalTypes()->isVoid());
+
+      auto &cs = solution.getConstraintSystem();
+
+      // Build an implicit empty tuple to represent `Void` return
+      auto *emptyTuple = TupleExpr::createEmpty(ctx,
+                                                /*LParenLoc=*/SourceLoc(),
+                                                /*RParenLoc=*/SourceLoc(),
+                                                /*Implicit=*/true);
+      emptyTuple->setType(ctx.TheEmptyTupleType);
+      // Cache the type of this new expression in the constraint system
+      // for the future reference.
+      cs.cacheExprTypes(emptyTuple);
+
+      returnStmt->setResult(emptyTuple);
+    }
 
     auto *resultExpr = returnStmt->getResult();