[Constraint solver] De-duplicate potential throw sites when merging partial solutions

Failure to deduplicate potential throw sites could cause these data
structures to grow expotentially. De-duplicate in the obvious way, noting
that we still need an effective data structure here.

Author: DougGregor

include/swift/Sema/ConstraintSystem.h

@@ -1466,6 +1466,15 @@ struct PotentialThrowSite {
 
   /// The locator that specifies where the throwing operation occurs.
   ConstraintLocator *locator;
+  
+  friend bool operator==(const PotentialThrowSite& lhs, const PotentialThrowSite &rhs) {
+    return lhs.kind == rhs.kind && lhs.type.getPointer() == rhs.type.getPointer() &&
+           lhs.locator == rhs.locator;
+  }
+  
+  friend bool operator!=(const PotentialThrowSite& lhs, const PotentialThrowSite &rhs) {
+    return !(lhs == rhs);
+  }
 };
 
 /// A complete solution to a constraint system.

lib/Sema/CSSolver.cpp

@@ -353,9 +353,21 @@ void ConstraintSystem::applySolution(const Solution &solution) {
       setCaseLabelItemInfo(info.first, info.second);
   }
 
-  potentialThrowSites.insert(potentialThrowSites.end(),
-                             solution.potentialThrowSites.begin(),
-                             solution.potentialThrowSites.end());
+  if (!potentialThrowSites.empty() && !solution.potentialThrowSites.empty()) {
+    DenseMap<void *, std::vector<PotentialThrowSite>> known;
+    for (const auto &throwSite : potentialThrowSites) {
+      known[throwSite.first.getOpaqueValue()].push_back(throwSite.second);
+    }
+      
+    for (const auto &throwSite : solution.potentialThrowSites) {
+      auto &sites = known[throwSite.first.getOpaqueValue()];
+      if (std::find(sites.begin(), sites.end(), throwSite.second) != sites.end())
+        continue;
+      
+      sites.push_back(throwSite.second);
+      potentialThrowSites.push_back(throwSite);
+    }
+  }
 
   for (auto param : solution.isolatedParams) {
     isolatedParams.insert(param);