where T: ?Sized doesn't parse

[jonas-schievink]

Example: http://is.gd/rB5WGc

RFC 490 says this should be accepted: https://github.com/rust-lang/rfcs/blob/master/text/0490-dst-syntax.md#detailed-design

Change the syntax for traits for dynamically-sized types to have a prefix ? instead of a postfix one:

trait Foo for ?Sized { ... }

Allow using this syntax in where clauses:

fn foo<T>(x: &T) where T: ?Sized { ... }

Note that, if the parser is changed to accept this, type collection needs to be fixed to look at where clauses instead of just type param bounds when figuring out if a Sized bound should be added.

[Stebalien]

Duplicate of #20503