[Type checker] Loosen an assertion to cope with multiple pre-checks

Function builders perform the expression "pre-check" operation
multiple times, which means that nested closures might have a
more-nested DeclContext than PreCheck expects. Loosen this assertion
to cope with that change.

(cherry picked from commit 79c5706)

Author: DougGregor

lib/Sema/TypeCheckConstraints.cpp

@@ -1392,7 +1392,9 @@ bool PreCheckExpression::walkToClosureExprPre(ClosureExpr *closure) {
 
   // Update the current DeclContext to be the closure we're about to
   // recurse into.
-  assert(DC == closure->getParent() && "Decl context isn't correct");
+  assert((closure->getParent() == DC ||
+          closure->getParent()->isChildContextOf(DC)) &&
+      "Decl context isn't correct");
   DC = closure;
   return true;
 }