[clang][dataflow] Fix MapLattice::insert() to not drop return value

[li.zhe.hua]

Summary

Fix MapLattice API to return std::pair<iterator, bool>,
allowing users to detect when an element has been inserted without
performing a redundant map lookup.

clang/include/clang/Analysis/FlowSensitive/MapLattice.h

public:
 
   MapLattice() = default;
 
   explicit MapLattice(Container C) { C = std::move(C); }
 
   // The `bottom` element is the empty map.
   static MapLattice bottom() { return MapLattice(); }
 
-  void insert(const std::pair<const key_type, mapped_type> &P) { C.insert(P); }
+  std::pair<iterator, bool>
+  insert(const std::pair<const key_type, mapped_type> &P) {
+    return C.insert(P);
+  }
 
-  void insert(std::pair<const key_type, mapped_type> &&P) {
-    C.insert(std::move(P));
+  std::pair<iterator, bool> insert(std::pair<const key_type, mapped_type> &&P) {
+    return C.insert(std::move(P));
   }
 
   unsigned size() const { return C.size(); }
   bool empty() const { return C.empty(); }
 
   iterator begin() { return C.begin(); }
   iterator end() { return C.end(); }
   const_iterator begin() const { return C.begin(); }

clang/unittests/Analysis/FlowSensitive/MapLatticeTest.cpp

private:
   bool Value;
 };
 } // namespace
 
 static constexpr int Key1 = 0;
 static constexpr int Key2 = 1;
 
 namespace {
+using ::testing::_;
 using ::testing::Pair;
 using ::testing::UnorderedElementsAre;
 
 TEST(MapLatticeTest, InsertWorks) {
   MapLattice<int, BooleanLattice> Lattice;
-  Lattice.insert({Key1, BooleanLattice(false)});
-  Lattice.insert({Key2, BooleanLattice(false)});
+  EXPECT_THAT(Lattice.insert({Key1, BooleanLattice(false)}), Pair(_, true));
+  EXPECT_THAT(Lattice.insert({Key2, BooleanLattice(false)}), Pair(_, true));
+
+  // Insertion fails on collision.
+  EXPECT_THAT(Lattice.insert({Key1, BooleanLattice(false)}), Pair(_, false));
+  EXPECT_THAT(Lattice.insert({Key2, BooleanLattice(false)}), Pair(_, false));
 
   EXPECT_THAT(Lattice, UnorderedElementsAre(Pair(Key1, BooleanLattice(false)),
                                             Pair(Key2, BooleanLattice(false))));
 }
 
 TEST(MapLatticeTest, ComparisonWorks) {
   MapLattice<int, BooleanLattice> Lattice1;
   Lattice1.insert({Key1, BooleanLattice(true)});