Avoid extern/parameter name collisions in show-goto-functions/dump-c output

While the preceding patch in this series ensures that the internal
representation is consistent, plain-text output would still have collisions
between parameter names and globals declared extern, because parameters were
never renamed. We now detect conflicts between prettified names and global
variable names.

Also added a comment explaining a somewhat obscure bit of code.

Author: tautschnig

src/ansi-c/expr2c.cpp

@@ -89,7 +89,7 @@ void expr2ct::get_shorthands(const exprt &expr)
   find_symbols_sett symbols;
   find_symbols(expr, symbols);
 
-  // avoid renaming parameters
+  // avoid renaming parameters, if possible
   for(find_symbols_sett::const_iterator
       it=symbols.begin();
       it!=symbols.end();
@@ -103,7 +103,16 @@ void expr2ct::get_shorthands(const exprt &expr)
 
     irep_idt sh=id_shorthand(*it);
 
-    ns_collision[symbol->location.get_function()].insert(sh);
+    std::string func = id2string(*it);
+    func = func.substr(0, func.rfind("::"));
+
+    // if there is a global symbol of the same name as the shorthand (even if
+    // not present in this particular expression) then there is a collision
+    const symbolt *global_symbol;
+    if(!ns.lookup(sh, global_symbol))
+      sh = func + "$$" + id2string(sh);
+
+    ns_collision[func].insert(sh);
 
     if(!shorthands.insert(std::make_pair(*it, sh)).second)
       UNREACHABLE;
@@ -125,6 +134,8 @@ void expr2ct::get_shorthands(const exprt &expr)
 
     if(!has_collision)
     {
+      // if there is a global symbol of the same name as the shorthand (even if
+      // not present in this particular expression) then there is a collision
       const symbolt *symbol;
       has_collision=!ns.lookup(sh, symbol);
     }