Parsing `<<` is incorrect for closure types

[alexcrichton]

This is a valid type signature, but it fails to parse today.

Option<<'a>||>

Most closure parsing was fixed in #13268, but this is the remaining bits of #11209.

Nominating because #11209 was nominated. I don't believe this should block 1.0.

[pnkfelix]

This is a grammar issue, but cmr does point out that its not a backwards compatibility hazard since it just represents programs that fail to parse today (and that we could fix backwards compatibly tomorrow).

We'll keep it on the 1.0 milestone (since we would like to resolve these grammatical issues), but it is just P-high (not a backcompat-lang like #11209 was).