C++: Add test to check lambda parameters emptyness

cpp/ql/test/library-tests/lambdas/syntax/empty_declarator.expected

@@ -0,0 +1,5 @@
+| parameters.cpp:8:5:8:20 | [...](...){...} |
+| parameters.cpp:10:5:10:26 | [...](...){...} |
+| parameters.cpp:13:5:13:20 | [...](...){...} |
+| parameters.cpp:16:5:18:5 | [...](...){...} |
+| parameters.cpp:25:5:25:14 | [...](...){...} |

cpp/ql/test/library-tests/lambdas/syntax/empty_declarator.ql

@@ -0,0 +1,5 @@
+import cpp
+
+from LambdaExpression e
+where e.emptyParameterListIsExplicit()
+select e

cpp/ql/test/library-tests/lambdas/syntax/parameters.cpp

@@ -0,0 +1,26 @@
+void test_lambda_declarator() {
+    [=](int, float) { };
+
+    [](int x = 42) { };
+
+    [](int x) { };
+
+    []() mutable { };
+
+    []() [[nodiscard]] { };
+    [] [[nodiscard]] { };
+
+    []() -> void { };
+
+    int i;
+    [&i]() {
+        i += 1;
+    };
+
+    [&i] {
+        i += 1;
+    };
+
+    [] { };
+    [=] () { };
+}