rustc: implement a lint for publicly visible private types.

These are types that are in exported type signatures, but are not
exported themselves, e.g.

    struct Foo { ... }

    pub fn bar() -> Foo { ... }

will warn about the Foo.

Such types are not listed in documentation, and cannot be named outside
the crate in which they are declared, which is very user-unfriendly.

cc #10573
sync: Rename arc::Condvar to arc::ArcCondvar.

The sync submodule also has a `Condvar` type, and its reexport was
shadowing the `arc` type, making it crate-private.
Publicise types/add #[allow(visible_private_types)] to a variety of places.

There's a lot of these types in the compiler libraries, and a few of the
older or private stdlib ones. Some types are obviously meant to be
public, others not so much.
auto merge of #12595 : huonw/rust/pub-vis-typ, r=alexcrichton

These are types that are in exported type signatures, but are not
exported themselves, e.g.

    struct Foo { ... }

    pub fn bar() -> Foo { ... }

will warn about the Foo.

Such types are not listed in documentation, and cannot be named outside
the crate in which they are declared, which is very user-unfriendly.

cc #10573.