Do not create locations with a set-but-empty function

The C front-end created symbols that had the function set to an empty
string. This was not accurately represented in the JSON representation,
but also shouldn't be used at all. Thus, changing the parser rather than
the JSON serialisation.

Author: tautschnig

src/ansi-c/c_preprocess.cpp

@@ -127,7 +127,7 @@ static void error_parse_line(
     if(state==2)
     {
       saved_error_location.set_file(file);
-      saved_error_location.set_function(irep_idt());
+      saved_error_location.clear_function();
       saved_error_location.set_line(line_no);
       saved_error_location.set_column(irep_idt());
     }

src/ansi-c/parser.y

@@ -3026,7 +3026,7 @@ function_definition:
           PARSER.pop_scope();
 
           // We are no longer in any function.
-          PARSER.set_function(irep_idt());
+          PARSER.clear_function();
         }
         ;
 
@@ -3589,7 +3589,7 @@ parameter_postfixing_abstract_declarator:
           // Clear function name in source location after parsing if
           // at global scope.
           if (PARSER.current_scope().prefix.empty()) {
-            PARSER.set_function(irep_idt());
+            PARSER.clear_function();
           }
 
           $$ = merge($4, $1);
@@ -3621,7 +3621,7 @@ parameter_postfixing_abstract_declarator:
           // Clear function name in source location after parsing if
           // at global scope.
           if (PARSER.current_scope().prefix.empty()) {
-            PARSER.set_function(irep_idt());
+            PARSER.clear_function();
           }
 
           if(parser_stack($5).is_not_nil())

src/util/parser.h

@@ -135,6 +135,11 @@ class parsert
     _source_location.set_function(function);
   }
 
+  void clear_function()
+  {
+    _source_location.clear_function();
+  }
+
   void advance_column(unsigned token_width)
   {
     column+=token_width;

src/util/source_location.h

@@ -134,9 +134,15 @@ class source_locationt:public irept
   DEPRECATED(SINCE(2022, 10, 13, "use identifier of containing function"))
   void set_function(const irep_idt &function)
   {
+    PRECONDITION(!function.empty());
     set(ID_function, function);
   }
 
+  void clear_function()
+  {
+    remove(ID_function);
+  }
+
   void set_property_id(const irep_idt &property_id)
   {
     set(ID_property_id, property_id);