Parse angle bracket terminating a const generic default value expr

[dtolnay]

As seen in the rustc test suite's src/test/ui/const-generics/defaults/auxiliary/const_defaulty.rs.

error: expected expression
 --> dev/main.rs:8:39
  |
8 | pub struct Defaulted<const N: usize=3>;
  |                                       ^

It appears we are parsing 3 > ??? as a binary operator expression.