Implement trace for PhantomData

[Narfinger]

PhantomData will be used in splitting the script crate in servo but this needs PhantomData to be traceable.
Currently the #[no_trace] part of the macro breaks a generic parameter, so

struct Foo<D> {
    #[no_trace]
    PhantomData<D>
}

does not recognize the type parameter D.

Testing: This does not need testing as phantomdata does not exist, so tracing it with an empty trace is fine.
Servo PR: Link to a companion pull request in the Servo repository. Remove this line if there is no companion PR.

[sagudev]

Currently the #[no_trace] part of the macro breaks a generic parameter,

If you do #[no_trace = "explenation"] it should avoid doing traceable check so it should work.