libsyntax: don't allow enum structs with no fields

Unit-like structs are written as `struct Foo;`, but we erroneously
accepted `struct Foo();` and took it to mean the same thing. Now we
don't, so use the `struct Foo;` form!

[breaking-change]
auto merge of #15098 : ben0x539/rust/nullary-tuple-struct, r=pcwalton

Reject `struct Foo();` to fix #15095.