[ConstraintSystem] Convert DefaultConstraints to a set vector to avoid duplicates

It's always been the case that partial solutions introduce
some storage duplication when applied back to the constraint
system to form a more complete solution with outer context,
 but the constraint systems used to be small before
introduction of result builders (and now multi-statement
inference), which make the duplication more visible.

Author: xedin

include/swift/Sema/ConstraintSystem.h

@@ -2450,7 +2450,7 @@ class ConstraintSystem {
   llvm::SmallMapVector<ASTNode, SmallVector<AppliedPropertyWrapper, 2>, 4> appliedPropertyWrappers;
 
   /// The locators of \c Defaultable constraints whose defaults were used.
-  std::vector<ConstraintLocator *> DefaultedConstraints;
+  llvm::SetVector<ConstraintLocator *> DefaultedConstraints;
 
   /// A cache that stores the @dynamicCallable required methods implemented by
   /// types.

lib/Sema/CSBindings.cpp

@@ -1983,7 +1983,7 @@ bool TypeVariableBinding::attempt(ConstraintSystem &cs) const {
 
   // If this was from a defaultable binding note that.
   if (Binding.isDefaultableBinding()) {
-    cs.DefaultedConstraints.push_back(srcLocator);
+    cs.DefaultedConstraints.insert(srcLocator);
 
     if (type->isPlaceholder() && reportHole())
       return true;

lib/Sema/CSSolver.cpp

@@ -259,8 +259,7 @@ void ConstraintSystem::applySolution(const Solution &solution) {
   }
 
   // Register the defaulted type variables.
-  DefaultedConstraints.insert(DefaultedConstraints.end(),
-                              solution.DefaultedConstraints.begin(),
+  DefaultedConstraints.insert(solution.DefaultedConstraints.begin(),
                               solution.DefaultedConstraints.end());
 
   // Add the node types back.
@@ -406,6 +405,12 @@ void truncate(llvm::SmallMapVector<K, V, N> &map, unsigned newSize) {
     map.pop_back();
 }
 
+template <typename V>
+void truncate(llvm::SetVector<V> &vector, unsigned newSize) {
+  while (vector.size() > newSize)
+    vector.pop_back();
+}
+
 } // end anonymous namespace
 
 ConstraintSystem::SolverState::SolverState(
@@ -542,8 +547,7 @@ ConstraintSystem::SolverScope::~SolverScope() {
     return;
 
   // Erase the end of various lists.
-  while (cs.TypeVariables.size() > numTypeVariables)
-    cs.TypeVariables.pop_back();
+  truncate(cs.TypeVariables, numTypeVariables);
 
   truncate(cs.ResolvedOverloads, numResolvedOverloads);