Language files: add support for a generic fallback provider

This adds a mechanism for asking languaget instances to produce functions they have not advertised.
DeepTest currently uses it to supply stub function bodies (jbmc replaces the calls, but this results
in a less-useful trace, as it lacks FUNCTION_CALL steps for stubbed functions).

Author: smowton

src/langapi/language.h

@@ -86,6 +86,22 @@ class languaget:public messaget
     const irep_idt &function_id, symbol_table_baset &symbol_table)
   { }
 
+  /// Can this languaget instance provide a body for `function_id`, which no
+  /// languaget instance has claimed to be able to provide?
+  virtual bool can_convert_fallback_method(const irep_idt &function_id) const
+  {
+    return false;
+  }
+
+  /// Populate the body of `function_id`, which no languaget instance has
+  /// claimed to be able to provide.
+  virtual bool convert_fallback_method(
+    const irep_idt &function_id, symbol_table_baset &symbol_table)
+  {
+    PRECONDITION(can_convert_fallback_method(function_id));
+    return false;
+  }
+
   /// Final adjustments, e.g. initializing stub functions and globals that
   /// were discovered during function loading
   virtual bool final(symbol_table_baset &symbol_table);

src/langapi/language_file.h

@@ -72,6 +72,9 @@ class language_filest:public messaget
   typedef std::map<irep_idt, language_filet *> lazy_method_mapt;
   lazy_method_mapt lazy_method_map;
 
+  typedef std::vector<language_filet *> fallback_method_providerst;
+  fallback_method_providerst fallback_method_providers;
+
 public:
   language_filet &add_file(const std::string &filename)
   {
@@ -126,11 +129,29 @@ class language_filest:public messaget
     lazy_method_mapt::iterator it=lazy_method_map.find(id);
     if(it!=lazy_method_map.end())
       it->second->convert_lazy_method(id, symbol_table);
+
+    for(const auto &file : file_map)
+    {
+      if(file.second.language->can_convert_fallback_method(id) &&
+         file.second.language->convert_fallback_method(id, symbol_table))
+      {
+        return;
+      }
+    }
   }
 
   bool can_convert_lazy_method(const irep_idt &id) const
   {
-    return lazy_method_map.count(id) != 0;
+    if(lazy_method_map.count(id) != 0)
+      return true;
+
+    for(const auto &file : file_map)
+    {
+      if(file.second.language->can_convert_fallback_method(id))
+        return true;
+    }
+
+    return false;
   }
 
   void clear()