Set a property id for recursion unwinding assertions

Previously we displayed an empty id.

regression/cbmc/unwinding_assertions1/main.c

@@ -0,0 +1,9 @@
+void foo()
+{
+  foo();
+}
+
+int main()
+{
+  foo();
+}

regression/cbmc/unwinding_assertions1/test.desc

@@ -0,0 +1,8 @@
+CORE
+main.c
+--unwind 3 --unwinding-assertions
+^EXIT=10$
+^SIGNAL=0$
+^VERIFICATION FAILED$
+^\[foo.recursion\]
+--

src/goto-symex/symex_target_equation.cpp

@@ -988,6 +988,12 @@ irep_idt symex_target_equationt::SSA_stept::get_property_id() const
     property_id = id2string(source.pc->source_location.get_function()) +
                   ".unwind." + std::to_string(source.pc->loop_number);
   }
+  else if(source.pc->is_function_call())
+  {
+    // this is likely a recursion unwinding assertion
+    property_id =
+      id2string(source.pc->source_location.get_function()) + ".recursion";
+  }
   else
   {
     // return empty