eachindex for linear indexing for Array and Memory: typeassert

The method return type is known concretely as `OneTo{Int}`, however the
compiler can't tell because there are too many methods matching
`length(::Array)`, over the world-splitting threshold, `max_methods`.

Cross-reference WIP PR JuliaLang#57627, which might help here in another way by
decreasing the number of unnecessary methods, however merging that PR
would also cause regressions if world-splitting is not disabled first
for `length` (xref WIP PR JuliaLang#59377).

Fix that by asserting the return type of the `length` call as `Int`.

The `typeassert` should improve abstract return type inference, and
consequently make the sysimage more resistant to invalidation.

Author: nsajko

base/abstractarray.jl

@@ -387,7 +387,7 @@ function eachindex(A::AbstractArray, B::AbstractArray...)
     @inline
     eachindex(IndexStyle(A,B...), A, B...)
 end
-eachindex(::IndexLinear, A::Union{Array, Memory}) = unchecked_oneto(length(A))
+eachindex(::IndexLinear, A::Union{Array, Memory}) = unchecked_oneto(length(A)::Int)
 eachindex(::IndexLinear, A::AbstractArray) = (@inline; oneto(length(A)))
 eachindex(::IndexLinear, A::AbstractVector) = (@inline; axes1(A))
 function eachindex(::IndexLinear, A::AbstractArray, B::AbstractArray...)