[Constraint system] Use hash table lookup appropriately.

It’s pointlessly inefficient to do an O(n) scan through a hash table to match
keys!

Author: DougGregor

lib/Sema/CSApply.cpp

@@ -7538,13 +7538,7 @@ ProtocolConformanceRef Solution::resolveConformance(
 }
 
 Type Solution::getType(const Expr *expr) const {
-  auto result = llvm::find_if(
-      nodeTypes, [&](const std::pair<TypedNode, Type> &node) -> bool {
-        if (auto *e = node.first.dyn_cast<const Expr *>())
-          return expr == e;
-        return false;
-      });
-
+  auto result = nodeTypes.find(expr);
   if (result != nodeTypes.end())
     return result->second;
 

lib/Sema/ConstraintSystem.h

@@ -1484,7 +1484,7 @@ class ConstraintSystem {
   /// nodes themselves. This allows us to typecheck and
   /// run through various diagnostics passes without actually mutating
   /// the types on the nodes.
-  llvm::DenseMap<TypedNode, Type> NodeTypes;
+  llvm::MapVector<TypedNode, Type> NodeTypes;
   llvm::DenseMap<std::pair<const KeyPathExpr *, unsigned>, TypeBase *>
       KeyPathComponentTypes;