RFC: Make dllimport/dllexport imply default visibility

[Ross Ridge]

Danny Smith writes:
>It doesn't currently.  However, the attribute would be useful on Window
>to avoid having to maintain an ugly hardcoded list of "default_excludes"
>in ld code. 

I don't know if overloading visibility attribute is a good idea here.
I think it would be a better idea to create a new attribute for this
feature.

					Ross Ridge