Constructors for structs with unions are incorrect

[christiangnrd]

#540 should fix the crash, but the constructors are not valid.

With PR #540 checked out (and the second struct commented out), the constructor takes no arguments:

julia> using Clang.Generators

julia> options = Dict("codegen" => Dict{String, Any}("add_record_constructors" => true))
Dict{String, Dict{String, Any}} with 1 entry:
  "codegen" => Dict("add_record_constructors"=>true)

julia> ctx = create_context(joinpath(@__DIR__, "test/include/constructors.h"),
                                get_default_args(), options)
Info: Parsing headers...
Context(...)

julia> build!(ctx)
[ Info: Processing header: /Users/christian/.julia/dev/Clang/test/include/constructors.h
[ Info: Building the DAG...
┌ Warning: default libname: ":libxxx" is being used, did you forget to set `library_name` in the toml file? It's safe to ignore this warning if you are using `library_names` as an exhaustive list (if so, set `library_name` to a dummy value to supress it).
└ @ Clang.Generators ~/.julia/dev/Clang/src/generator/audit.jl:16
[ Info: Emit Julia expressions...
struct _PackedFloat3
    data::NTuple{12, UInt8}
end

function Base.getproperty(x::Ptr{_PackedFloat3}, f::Symbol)
    f === :x && return Ptr{Cfloat}(x + 0)
    f === :y && return Ptr{Cfloat}(x + 4)
    f === :z && return Ptr{Cfloat}(x + 8)
    f === :elements && return Ptr{NTuple{3, Cfloat}}(x + 0)
    return getfield(x, f)
end

function Base.getproperty(x::_PackedFloat3, f::Symbol)
    r = Ref{_PackedFloat3}(x)
    ptr = Base.unsafe_convert(Ptr{_PackedFloat3}, r)
    fptr = getproperty(ptr, f)
    GC.@preserve r unsafe_load(fptr)
end

function Base.setproperty!(x::Ptr{_PackedFloat3}, f::Symbol, v)
    unsafe_store!(getproperty(x, f), v)
end

function _PackedFloat3()
    ref = Ref{_PackedFloat3}()
    ptr = Base.unsafe_convert(Ptr{_PackedFloat3}, ref)
    ref[]
end

const PackedFloat3 = _PackedFloat3

[ Info: Done!
Context(...)

With PR #540 checked out, the second struct commented out, and line 438 of codegen.jl uncommented, the constructor takes all arguments:

julia> using Clang.Generators
Precompiling Clang...
  1 dependency successfully precompiled in 3 seconds. 28 already precompiled.
o
julia> options = Dict("codegen" => Dict{String, Any}("add_record_constructors" => true))
Dict{String, Dict{String, Any}} with 1 entry:
  "codegen" => Dict("add_record_constructors"=>true)

julia> ctx = create_context(joinpath(@__DIR__, "test/include/constructors.h"),
                                get_default_args(), options)
[ Info: Parsing headers...
Context(...)

julia> build!(ctx)
[ Info: Processing header: /Users/christian/.julia/dev/Clang/test/include/constructors.h
[ Info: Building the DAG...
┌ Warning: default libname: ":libxxx" is being used, did you forget to set `library_name` in the toml file? It's safe to ignore this warning if you are using `library_names` as an exhaustive list (if so, set `library_name` to a dummy value to supress it).
└ @ Clang.Generators ~/.julia/dev/Clang/src/generator/audit.jl:16
[ Info: Emit Julia expressions...
struct _PackedFloat3
    data::NTuple{12, UInt8}
end

function Base.getproperty(x::Ptr{_PackedFloat3}, f::Symbol)
    f === :x && return Ptr{Cfloat}(x + 0)
    f === :y && return Ptr{Cfloat}(x + 4)
    f === :z && return Ptr{Cfloat}(x + 8)
    f === :elements && return Ptr{NTuple{3, Cfloat}}(x + 0)
    return getfield(x, f)
end

function Base.getproperty(x::_PackedFloat3, f::Symbol)
    r = Ref{_PackedFloat3}(x)
    ptr = Base.unsafe_convert(Ptr{_PackedFloat3}, r)
    fptr = getproperty(ptr, f)
    GC.@preserve r unsafe_load(fptr)
end

function Base.setproperty!(x::Ptr{_PackedFloat3}, f::Symbol, v)
    unsafe_store!(getproperty(x, f), v)
end

function _PackedFloat3(x::Cfloat, y::Cfloat, z::Cfloat, elements::NTuple{3, Cfloat})
    ref = Ref{_PackedFloat3}()
    ptr = Base.unsafe_convert(Ptr{_PackedFloat3}, ref)
    ptr.x = x
    ptr.y = y
    ptr.z = z
    ptr.elements = elements
    ref[]
end

const PackedFloat3 = _PackedFloat3

[ Info: Done!
Context(...)

Should we try to generate all possible constructors by checking which fields don't overlap or should we disable constructors for these cases? For example, the struct above would generate a constructor with signature _PackedFloat3(x::Cfloat, y::Cfloat, z::Cfloat), and another with signature _PackedFloat3(elements::NTuple{3, Cfloat}). I can see this getting out of hand for more complicated unions though.