abi: make dig_scalar_pointee search for offsets in all layout fields.

[eddyb]

This allows representing types like core::ops::Bound<&u32>, which would've previously caused a fatal error:

error: dig_scalar_pointee_adt Variants::Multiple TagEncoding::Direct makes no sense: TyAndLayout {
    ty: core::ops::Bound<&u32>,
    layout: Layout {

(rest of the output elided - it's just the Layout of that type)

The motivation for this is updating past rust-lang/rust#87723 - sadly, that kind of code isn't supported yet (I guess we'd have to have a transformation to get rid of the pointer on the stack, for it to work), but at the very least it results in a (validation) error only if it's called, there's no fatal error while building core, anymore.