C++: Add additional test cases for explict and explicit(bool) specifiers

Note that the `explict(bool)` specifiers currently do not end up in the
database.

cpp/ql/test/library-tests/specifiers2/cpp20.cpp

@@ -7,4 +7,27 @@ class TestConstexpr {
     constexpr int member_const_constexpr() const { return 0; }
 };
 
+struct TestExplict {
+    explicit TestExplict();
+};
+
+template<typename T>
+struct TestExplicitBool {
+    explicit(sizeof(T) == 1)
+    TestExplicitBool(const T);
+};
+
+explicit TestExplicitBool(const double)  -> TestExplicitBool<int>;
+
+template<typename T>
+explicit(sizeof(T) == 4)
+TestExplicitBool(const T)  -> TestExplicitBool<int>;
+
+void TestExplicitBoolFun() {
+    new TestExplicitBool<char>(0);
+    new TestExplicitBool<int>(0);
+    new TestExplicitBool(0.0f);
+    new TestExplicitBool(0.0);
+}
+
 } // namespace cpp20