Refine eltype of Base.Generator when its type is concrete

[Tortar]

Tries to address #54157

Now:

julia> iter = (x^2 for x in 1:10)
Base.Generator{UnitRange{Int64}, var"#1#2"}(var"#1#2"(), 1:10)

julia> eltype(iter)
Int64

Before

julia> iter = (x^2 for x in 1:10)
Base.Generator{UnitRange{Int64}, var"#1#2"}(var"#1#2"(), 1:10)

julia> eltype(iter)
Any

Took inspiration from Base.@default_eltype for the infer_type macro. The first couldn't be used directly because it accepts an instance, not a type.