C/C++ front-end: void-typed symbols are permitted when extern

This leaves it to the linker to resolve to some other type, and is
quietly accepted by GCC. Required for building the Linux kernel. The
changes in 9018306 were overly strict.

Author: tautschnig

regression/ansi-c/invalid_use_of_void1/main.c

@@ -1,3 +1,5 @@
+extern void this_is_ok;
+
 struct a
 {
 #ifdef void_member

src/ansi-c/c_typecheck_base.cpp

@@ -103,7 +103,7 @@ void c_typecheck_baset::typecheck_symbol(symbolt &symbol)
     symbol.pretty_name=new_name;
   }
 
-  if(!symbol.is_type && symbol.type.id() == ID_empty)
+  if(!symbol.is_type && !symbol.is_extern && symbol.type.id() == ID_empty)
   {
     error().source_location = symbol.location;
     error() << "void-typed symbol not permitted" << eom;

src/cpp/cpp_typecheck_code.cpp

@@ -448,7 +448,7 @@ void cpp_typecheckt::typecheck_decl(codet &code)
     if(is_typedef)
       continue;
 
-    if(!symbol.is_type && symbol.type.id() == ID_empty)
+    if(!symbol.is_type && !symbol.is_extern && symbol.type.id() == ID_empty)
     {
       error().source_location = symbol.location;
       error() << "void-typed symbol not permitted" << eom;

src/cpp/cpp_typecheck_declaration.cpp

@@ -148,7 +148,7 @@ void cpp_typecheckt::convert_non_template_declaration(
       declaration_type, declaration.storage_spec(),
       declaration.member_spec(), declarator);
 
-    if(!symbol.is_type && symbol.type.id() == ID_empty)
+    if(!symbol.is_type && !symbol.is_extern && symbol.type.id() == ID_empty)
     {
       error().source_location = symbol.location;
       error() << "void-typed symbol not permitted" << eom;