Rollup merge of #36868 - petrochenkov:adtstab, r=nikomatsakis

Partially stabilize RFC 1506 "Clarify relationships between ADTs"

Lifted restrictions on tuple structs/variants are stabilized, i.e. `S{..}` can be used with any structs and empty tuple structs are permitted without feature gate.
Numeric fields in struct expressions/patterns `S { 0: a, 1: b }` are **NOT** stabilized.
This was implemented 1.5 months ago in Rust 1.12, but this is a tiny technical change that could probably go even without RFC/stabilization period.

cc https://github.com/rust-lang/rust/issues/35626 https://github.com/rust-lang/rust/pull/36871
r? @nikomatsakis