Sema: Tweak SanitizeExpr handling of AutoClosureExprs

Curry thunks create AutoClosureExprs with parameters, and we can't just
fold those away.

Also, remove an older AutoClosureExpr cleanup that seems to be
redundant.

Author: slavapestov

lib/Sema/CSGen.cpp

@@ -3409,8 +3409,11 @@ namespace {
 
         // Restore '@autoclosure'd value.
         if (auto ACE = dyn_cast<AutoClosureExpr>(expr)) {
-          expr = ACE->getSingleExpressionBody();
-          continue;
+          // This is only valid if the closure doesn't have parameters.
+          if (ACE->getParameters()->size() == 0) {
+            expr = ACE->getSingleExpressionBody();
+            continue;
+          }
         }
 
         // Remove any semantic expression injected by typechecking.

lib/Sema/TypeCheckConstraints.cpp

@@ -1229,13 +1229,6 @@ namespace {
         DC = ce->getParent();
       }
 
-      // Strip off any AutoClosures that were produced by a previous type check
-      // so that we don't choke in CSGen.
-      // FIXME: we shouldn't double typecheck, but it looks like code completion
-      // may do so in some circumstances. rdar://21466394
-      if (auto autoClosure = dyn_cast<AutoClosureExpr>(expr))
-        return autoClosure->getSingleExpressionBody();
-
       // A 'self.init' or 'super.init' application inside a constructor will
       // evaluate to void, with the initializer's result implicitly rebound
       // to 'self'. Recognize the unresolved constructor expression and