Add warning about unsound simplification

At the time of adding this commit, this simplification is known to
return unsound/erroneous results for input programs featuring recursion.
This warning should be remove once the known issues have been fixed. See
#7041

Author: thomasspriggs

regression/goto-analyzer-simplify/soundness-warning/main.c

@@ -0,0 +1,4 @@
+
+void main()
+{
+}

regression/goto-analyzer-simplify/soundness-warning/test.desc

@@ -0,0 +1,7 @@
+CORE
+main.c
+--vsd --vsd-values constants
+\*\*\*\* WARNING\: This simplification is currently known to be unsound for input programs which include recursion\. Do not use it if soundness is important for your use case\.
+^SIGNAL=0$
+^EXIT=0$
+--

src/goto-analyzer/static_simplifier.cpp

@@ -56,6 +56,10 @@ bool static_simplifier(
 
   messaget m(message_handler);
   m.status() << "Simplifying program" << messaget::eom;
+  m.warning() << "**** WARNING: This simplification is currently known to be "
+                 "unsound for input programs which include recursion. Do not "
+                 "use it if soundness is important for your use case."
+              << messaget::eom;
 
   for(auto &gf_entry : goto_model.goto_functions.function_map)
   {