C++: Add getTemplateClass to DeductionGuide

cpp/ql/test/library-tests/deduction_guides/test.cpp

@@ -0,0 +1,18 @@
+// semmle-extractor-options: -std=c++20
+
+template<typename T>
+struct C {
+    C(const T);
+};
+
+C(const double)  -> C<int>;
+
+template<typename T>
+C(const T)  -> C<int>;
+
+void test() {
+    new C<char>(0);
+    new C<int>(0);
+    new C(0.0f);
+    new C(0.0);
+}