Disallow return types of `&'static foo` from being written as `&foo`

[bstrie]

As far as I can tell, when &str appears as the return type of a function, this silently implies &'static str. I think this holds for all other types as well, not just str. I propose that we force people to be explicit here, since our general rule is that any borrowed pointer that is returned from a function must have an explicit lifetime.

[bluss]

The buglet in pull #8308 was found while discussing this issue

[bstrie]

Nominating for Well-Defined.

[nikomatsakis]

I think &str should be the same as returning any other borrowed pointer? If it's not, that's a bug, but otherwise this seems like a dup of #2478.

[bluss]

It's a duplicate.

[catamorphism]

Closing as dup