in `Foo(X)` dep-nodes, allow X to be a `ty` not a `tt`

Before, the identifier `X` was also used when generating a pattern
to match against the dep-node. So `Foo(DefId)` would generate a match
pattern like:

    match foo {
        Foo(DefId) => ...
    }

This does not scale to more general types like `&'tcx
Ty<'tcx>`. Therefore, we now require *exactly one* argument (the macro
was internally tupling anyway, and no actual nodes use more than one
argument), and then we can generate a fixed pattern like:

    match foo {
        Foo(arg) => ...
    }

Huzzah. (Also, hygiene is nice.)

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