cconvert of map(pointer, String[...]) to Ptr{Ptr{Cchar}} fails on nightly

[vchuravy]

Found during CI on https://github.com/cesmix-mit/LAMMPS.jl

 ERROR: LoadError: TypeError: in RefArray, in A, expected A<:(AbstractArray{Ptr{Int8}}), got Type{Vector{Ptr{UInt8}}}
Stacktrace:
 [1] Ref{Ptr{Int8}}(a::Vector{Ptr{UInt8}})
   @ Base ./refpointer.jl:158
 [2] cconvert(::Type{Ptr{Ptr{Int8}}}, a::Vector{Ptr{UInt8}})
   @ Base ./refpointer.jl:176
 [3] lammps_open_no_mpi(argc::Int64, argv::Vector{Ptr{UInt8}}, ptr::Ptr{Nothing})
   @ LAMMPS.API ~/work/LAMMPS.jl/LAMMPS.jl/src/api.jl:53
 [4] LMP(args::Vector{String}, comm::Nothing)
   @ LAMMPS ~/work/LAMMPS.jl/LAMMPS.jl/src/LAMMPS.jl:69

The ccall is generated by Clang.jl and takes as an argument a Ptr{Ptr{Cchar}}

https://github.com/cesmix-mit/LAMMPS.jl/blob/126d9711e37fef035d5c7327d1181daf12b7b22a/src/api.jl#L53

and while the argsv = map(pointer, args) should probably be inside the GC.@preserve

https://github.com/cesmix-mit/LAMMPS.jl/blob/126d9711e37fef035d5c7327d1181daf12b7b22a/src/LAMMPS.jl#L59

this used to work.

[vtjnash]

Seems like it being a bit too strict now, while Int8 vs UInt8 would be an error in C, but usually it is allowed in Julia since the ABI is not affected by it (and the preserve and pointer calls are not necessary, as this call probably would have already worked if the user hadn't gone through that trouble)