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]
auto merge of #15062 : pcwalton/rust/trailing-plus, r=brson

This will break code that looks like `Box<Trait+>`. Change that code to
`Box<Trait>` instead.

Closes #14925.

[breaking-change]

r? @brson