Allow reinterpret(Union{Missing, T}, ::Vector{Union{Missing, U}})

[cstjean]

This works:

julia> v = [1,2,3,4]
4-element Array{Int64,1}:
 1
 2
 3
 4

julia> reinterpret(Some{Int64}, v)
4-element reinterpret(Some{Int64}, ::Array{Int64,1}):
 1
 2
 3
 4

It would be very nice if this could be supported, too:

julia> v2 = [1,2,3,4,missing]
5-element Array{Union{Missing, Int64},1}:
 1       
 2       
 3       
 4       
  missing

julia> reinterpret(Union{Missing, Some{Int64}}, v)
ERROR: ArgumentError: cannot reinterpret `Int64` `Union{Missing, Some{Int64}}`, type `Union{Missing, Some{Int64}}` is not a bits type
Stacktrace:
 [1] (::getfield(Base, Symbol("#throwbits#200")))(::Type{Int64}, ::Type{Union{Missing, Some{Int64}}}, ::Type{Union{Missing, Some{Int64}}}) at ./reinterpretarray.jl:16
 [2] reinterpret(::Type{Union{Missing, Some{Int64}}}, ::Array{Int64,1}) at ./reinterpretarray.jl:33
 [3] top-level scope at none:0

I've explained the use case on discourse. Briefly, when working with Unitful vectors, one often has to take out the units (eg. to make a linear regression), then tack them back on the result. Both operations could conceivably be done allocation-free with reinterpret, but are very slow on vectors that contain missings.

[nalimilan]

See #26681 about a similar issue with convert.