C++: Add support for C++ requires expressions

cpp/downgrades/21afcc6cd936ccf0d53a6a669e992d695ca82246/exprs.ql

@@ -0,0 +1,17 @@
+class Expr extends @expr {
+  string toString() { none() }
+}
+
+class Location extends @location_expr {
+  string toString() { none() }
+}
+
+predicate isExprRequirement(Expr expr) {
+  exists(int kind | exprs(expr, kind, _) | kind = [391, 392, 393])
+}
+
+from Expr expr, int kind, int kind_new, Location location
+where
+  exprs(expr, kind, location) and
+  if isExprRequirement(expr) then kind_new = 1 else kind_new = kind
+select expr, kind_new, location