infinite recursion in type_structurally_contains with enums

[comex]

Test case:

pub enum Global {
    GVar(Type)
}

pub enum Type {
    TNamed(Type)
}

fn main() {}

% rustc /tmp/test.rs           
task 'rustc' has overflowed its stack
zsh: illegal hardware instruction  rustc /tmp/test.rs

Tested with 60260b6.

[huonw]

Thanks for the report: looks like this is a dupe of #3008.