Fix accidental recursion in jl_concrete_subtype

[Keno]

jl_concrete_subtype was recursing into the vararg element of a tuple,
which I don't think is correct (because it could be zero-length and
thus be constructible even if the element type is not). Fix that and
as a bonus get we are now able to get rid of the previous condition
that (accidentally) special cased tuples to have that behavior.

Fixes #31062

[Keno]

Well, I guess we found out what that condition is for. I suppose it's not possible to create a type that contains itself without either unions (which this returns true for), or having ninitialized < nfields.

[Keno]

Though I guess that only prevents us from instantiating such a type, not from defining it and indeed:

julia> struct Bar
       x::Bar
       end

julia> precompile(identity, (Bar,))
ERROR: StackOverflowError:

I suppose we could cache it during datatype initialization (which essentially gets us the recursion memoization for free by storing the flag in the datatype).

[vtjnash]

Yes, Bar there could make an excellent implementation for a Never type.

I started computing that property in datatype initialization, of whether the type is able to be allocated versus whether the fields could form a cycle, to set isinlinealloc (although as you may guess from the name, it's only currently computed for isbitstypes):

julia> Bar.isinlinealloc
false

[vtjnash]

Seems valid, so LGTM.