Implement -Wimplicit-fallthrough: core

[Joseph Myers]

On Fri, 12 Aug 2016, Marek Polacek wrote:

> Not sure if error is appropriate here, or whether I should downgrade the
> error to a warning and ignore the attribute.

I'd say the semantics for uses of attributes that currently parse should 
remain unchanged: they should not be reinterpreted (for non-fallthrough 
attributes) to be attributes on a statement rather than on an empty 
declaration.

-- 
Joseph S. Myers
joseph@codesourcery.com