libsyntax: Stop parsing `+` with no bounds after it.

This will break code that looks like `Box<Trait+>`. Change that code to
`Box<Trait>` instead.

Closes #14925.

[breaking-change]

r? @brson
Has the token stream here been modified? Because this is eating a `token::COLON` and then claiming in the error that it was a `+`.

It seems this should also be making `proc():` into an error, but [brson's comment](https://github.com/pcwalton/rust/commit/18dda34d72967a95d781dab79a9a8331472b925d#commitcomment-6750836) on the commit suggests that `proc():` is supposed to still be valid.