unions with only Abi::Scalar fields should get Abi::Scalar themselves

[RalfJung]

Consider these types:

union U1 { f: usize }
union U2 { f: *const i32 }
union U3 { f: usize, g: *const i32 }

The first two are getting scalar ABI, but the last one is an aggregate. (As shown by this debug output.)

It seems reasonable to also give the last union Scalar ABI, no? Of course it should be Scalar::Union, but still.

@scottmcm ran into this in #113344.
Cc @eddyb @oli-obk (as usual for layout issues ;)