move 'extern with initializer' validation after the validate method

move 'extern with initializer' validation after the validate method

Before the call to the method external_decl::validate_decl()
there is another validation done which check if the declaration
linkage is not external, otherwise an error is issued and the
'extern' is removed from the declaration.

While also valid for C99 for-loop initializer, this is less
desirable because in this context, 'extern' is invalid anyway
and removing it from the declaration make it imposible to issue
a diagnostic about it.

Fix this by moving the 'extern with initializer' check after the
call to validate_decl() method, where it is always pertinent and
so allowing process_for_loop_decl() to make its own diagnostic.

Signed-off-by: Luc Van Oostenryck <luc.vanoostenryck@gmail.com>
Signed-off-by: Christopher Li <sparse@chrisli.org>