[CSClosure] Use rewritten expression when applying solution to a brace statement

It went un-noticed because currently only single-statement closures
using this logic and their bodies are modeled as a single return statement.

Author: xedin

lib/Sema/CSClosure.cpp

@@ -181,7 +181,7 @@ class ClosureConstraintApplication
       if (auto expr = node.dyn_cast<Expr *>()) {
         // Rewrite the expression.
         if (auto rewrittenExpr = rewriteExpr(expr))
-          node = expr;
+          node = rewrittenExpr;
         else
           hadError = true;
       } else if (auto stmt = node.dyn_cast<Stmt *>()) {