C++: Add a skeleton class for requires expressions

cpp/ql/lib/semmle/code/cpp/Concept.qll

@@ -0,0 +1,14 @@
+/**
+ * Provides classes for working with C++ concepts.
+ */
+
+import semmle.code.cpp.exprs.Expr
+
+/**
+ * A C/C++ requires expression.
+ */
+class RequiresExpr extends Expr, @requires_expr {
+  override string toString() { result = "requires ..." }
+
+  override string getAPrimaryQlClass() { result = "RequiresExpr" }
+}