CS node->type map requires ordering because it can be partially undone,

but Solution node->type map is all or nothing, so can be cheaper.

Author: gregomni

lib/Sema/ConstraintSystem.h

@@ -1016,7 +1016,7 @@ class Solution {
   llvm::SmallPtrSet<ConstraintLocator *, 2> DefaultedConstraints;
 
   /// The node -> type mappings introduced by this solution.
-  llvm::MapVector<ASTNode, Type> nodeTypes;
+  llvm::DenseMap<ASTNode, Type> nodeTypes;
 
   /// Contextual types introduced by this solution.
   std::vector<std::pair<ASTNode, ContextualTypeInfo>> contextualTypes;