"unsupported representation for univariant enum" when using the `repr` attribute with single-variant enums

[bstrie]

Reproduce:

#[repr(u8)] enum Bar { Foo }
fn main() {}

Error:

repr.rs:1:13: 1:29 error: unsupported representation for univariant enum
repr.rs:1 #[repr(u8)] enum Bar { Foo }
                      ^~~~~~~~~~~~~~~~
error: aborting due to previous error

@cmr was convinced that this ought to be fixed. #rust also produced an example of where this would be necessary, from GLib:

typedef enum {
    G_THREAD_ERROR_AGAIN /* Resource temporarily unavailable */
} GThreadError;

[alexcrichton]

Closing as a dupe of #10292.