[PATCH] Fix externally_visible attribute handling (PR c/28744)

[Jan Hubicka]

> Hi!
> 
> As the attached testcases show, we have been warning and ignoring
> "externally_visible" attribute on extern variable decls (but accepting
> it on extern function prototypes).  This patch fixes it, on the other
> side adds the warning to cases which can't be caught already during
> parsing (e.g. GCC wasn't warning about
> static void foo (void) { }
> extern void foo (void) __attribute__((externally_visible));
> ).
> Ok for trunk?

OK, thanks!
Honza