Implement -Wimplicit-fallthrough: core

[Jeff Law]

On 08/12/2016 09:58 AM, Joseph Myers wrote:
> 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.
Agreed.

jeff