[ConstraintSystem] Don't allow explicit closure result to be implicitly converted to Void

It's allowed to convert a single statement closure from `(...) -> T` to `(...) -> Void`
_only_ if there is no explicit `return` in the body.

Resolves: [SR-12277](https://bugs.swift.org/browse/SR-12277)
Resolves: rdar://problem/52204414

lib/Sema/ConstraintSystem.cpp

@@ -3434,6 +3434,7 @@ void constraints::simplifyLocator(Expr *&anchor,
       }
       break;
 
+    case ConstraintLocator::ClosureBody:
     case ConstraintLocator::ClosureResult:
       if (auto CE = dyn_cast<ClosureExpr>(anchor)) {
         if (CE->hasSingleExpressionBody()) {