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types: compute self-referential supertypes lazily (#62847)

🤖 A struct definition whose supertype declaration recursively applies
the type being defined at structurally increasing parameters, such as

    struct S{T} <: A{S{Tuple{T}}} end

has an infinite supertype graph: eagerly instantiating each level's
supertype creates the next level. Today the definition-time partial
fixup truncates that recursion by silently caching wrong supertypes —
the same interned type can permanently report `Any` or its true
supertype depending on which expression happened to construct it first,
and `supertype` disagrees with `<:`.

With this PR, an instantiation created while another instantiation of
the same (supertype-self-referential) typename is in flight now defers
its supertype and completes it lazily, one level per demand
(`jl_datatype_compute_super`). Every level of the graph is correct and
independent of instantiation order. Same-parameter self-references still
tie their knot through the instantiation stack, so ordinary recursive
types are unaffected, and field types stay eager — a concrete type must
publish with its layout, so the field-type analogue of this recursion
(the `S1` case of #61347) is left for a follow-up.

Addresses the supertype half of #61347.

Assisted-by: Claude Code (Fable 5)

Co-authored-by: Keno Fischer <Keno@users.noreply.github.com>

Coverage Stats

13 of 17 new or added lines in 4 files covered. (76.47%)

24 existing lines in 8 files now uncovered.

69416 of 88616 relevant lines covered (78.33%)

23947628.23 hits per line