Auto merge of #36151 - camlorn:struct_layout_optimization, r=eddyb

Auto merge of #36151 - camlorn:struct_layout_optimization, r=eddyb

refactor to remove trans::adt and make rustc::ty::layout authoritative

I asked on IRC about optimizing struct layout by reordering fields from most-aligned to least-aligned and somehow ended up getting talked into doing this.  The goal here is to make `layout` authoritative and to remove `adt`.  The former has been accomplished by reimplementing `represent_type_uncached` and the latter is in progress.  @eddyb thought I should make the PR now.

My plan is to reserve the actual optimization for a second PR, as this work is useful by itself.