C/C++ front-ends: report a meaningful error for invalid function definition

The parser happily accepts the syntax shown in the regression test
included, but type checking strictly expects code instead of an
initializer list. Report a proper error message, like GCC does, instead
of failing an invariant.

Test generated using C-Reduce starting from an SV-COMP task.

Author: tautschnig

regression/ansi-c/invalid_function_definition1/main.c

@@ -0,0 +1 @@
+void a() = {};

regression/ansi-c/invalid_function_definition1/test.desc

@@ -0,0 +1,9 @@
+CORE test-c++-front-end
+main.c
+
+^EXIT=(64|1)$
+^SIGNAL=0$
+function 'a(\(\))?' is initialized with initializer_list
+^CONVERSION ERROR$
+--
+Invariant check failed

src/ansi-c/c_typecheck_base.cpp

@@ -498,9 +498,15 @@ void c_typecheck_baset::typecheck_redefinition_non_type(
 
 void c_typecheck_baset::typecheck_function_body(symbolt &symbol)
 {
-  code_typet &code_type=to_code_type(symbol.type);
+  if(symbol.value.id() != ID_code)
+  {
+    error().source_location = symbol.location;
+    error() << "function '" << symbol.name << "' is initialized with "
+            << symbol.value.id() << eom;
+    throw 0;
+  }
 
-  assert(symbol.value.is_not_nil());
+  code_typet &code_type = to_code_type(symbol.type);
 
   // reset labels
   labels_used.clear();

src/cpp/cpp_typecheck_function.cpp

@@ -91,6 +91,14 @@ void cpp_typecheckt::convert_function(symbolt &symbol)
   if(symbol.value.is_nil())
     return;
 
+  if(symbol.value.id() != ID_code)
+  {
+    error().source_location = symbol.location;
+    error() << "function '" << symbol.name << "' is initialized with "
+            << symbol.value.id() << eom;
+    throw 0;
+  }
+
   // enter appropriate scope
   cpp_save_scopet saved_scope(cpp_scopes);
   cpp_scopet &function_scope=cpp_scopes.set_scope(symbol.name);