Rustfmt eating the colon in bound-less where clause entries (for lifetimes).

```rust
pub fn foo<'a>()
where
    'a: ,
{
}
```
becomes
```rust
pub fn foo<'a>()
where
    'a,
{
}
```
resulting in syntax error.

This kind of code can actually be usefult to turn `'a` into an early bound lifetime.

Tested in the `rustfmt` shipped with nightly rust (`rustfmt 1.4.30-nightly (acd9486 2020-12-20)`).