Preview: PATCH to overhaul C++ visibility handling

[Jason Merrill]

Danny Smith wrote:
> class Foo
> {
>    void bar();
> }
> 
> void Foo::bar() {};
> 
> // A misplaced 'forward declaration' 
> class __attribute__ ((dllimport)) Foo; // warn and ignore?
> 
> When not ignored, the dllimport example in particular has caused trouble
> in past.

My patch gives an error for this with attribute visibility, but I'll 
extend it to all attributes.

Jason