[ConstraintSystem] Convert ImplicitValueConversions to a map vector to avoid duplicates

include/swift/Sema/ConstraintSystem.h

@@ -2399,7 +2399,7 @@ private:
 
   /// The set of implicit value conversions performed by the solver on
   /// a current path to reach a solution.
-  SmallVector<std::pair<ConstraintLocator *, ConversionRestrictionKind>, 2>
+  llvm::SmallMapVector<ConstraintLocator *, ConversionRestrictionKind, 2>
       ImplicitValueConversions;
 
   /// The worklist of "active" constraints that should be revisited

lib/Sema/CSSimplify.cpp

@@ -11322,7 +11322,7 @@ ConstraintSystem::simplifyRestrictedConstraintImpl(
                   FunctionType::get({FunctionType::Param(type1)}, type2),
                   memberTy, applicationLoc);
 
-    ImplicitValueConversions.push_back(
+    ImplicitValueConversions.insert(
         {getConstraintLocator(locator), restriction});
     return SolutionKind::Solved;
   }

lib/Sema/CSSolver.cpp

@@ -305,7 +305,7 @@ void ConstraintSystem::applySolution(const Solution &solution) {
   }
 
   for (auto &valueConversion : solution.ImplicitValueConversions) {
-    ImplicitValueConversions.push_back(valueConversion);
+    ImplicitValueConversions.insert(valueConversion);
   }
 
   // Register the argument lists.