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);
+}

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

@@ -0,0 +1,3 @@
+| file://:0:0:0:0 | C | test.cpp:4:8:4:8 | C<T> |
+| test.cpp:5:5:5:5 | (unnamed deduction guide) | test.cpp:4:8:4:8 | C<T> |
+| test.cpp:11:1:11:1 | C | test.cpp:4:8:4:8 | C<T> |

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

@@ -0,0 +1,4 @@
+import cpp
+
+from DeductionGuide d
+select d, d.getTemplateClass()