[clang][dataflow] Use `IntegerValue` instead of `StructValue` in `ValueTest`.

[mboehme]

Summary

Soon, it will no longer be possible to default-construct StructValue. For details, see https://discourse.llvm.org/t/70086.

For completeness, also add a test that areEquivalentValues() on different IntegerValues returns false.

clang/unittests/Analysis/FlowSensitive/ValueTest.cpp

 #include <memory>
 
 namespace {
 
 using namespace clang;
 using namespace dataflow;
 
 TEST(ValueTest, EquivalenceReflexive) {
-  StructValue V;
+  IntegerValue V;
   EXPECT_TRUE(areEquivalentValues(V, V));
 }
 
+TEST(ValueTest, DifferentIntegerValuesNotEquivalent) {
+  IntegerValue V1;
+  IntegerValue V2;
+  EXPECT_FALSE(areEquivalentValues(V1, V2));
+}
+
 TEST(ValueTest, AliasedReferencesEquivalent) {
   auto L = ScalarStorageLocation(QualType());
   ReferenceValue V1(L);
   ReferenceValue V2(L);
   EXPECT_TRUE(areEquivalentValues(V1, V2));
   EXPECT_TRUE(areEquivalentValues(V2, V1));
 }