Product of non-dense matrix problem

Product of two Triangular matrix:

```julia
using Tracker
using LinearAlgebra

X = randn(3, 3)

function g1(x)
y = UpperTriangular(x)
sum(y * y)
end

Tracker.gradient(g1, X)
```

throw:

```
MethodError: Cannot `convert` an object of type Array{Float64,2} to an object of type UpperTriangular{Float64,Array{Float64,2}}
Closest candidates are:
convert(::Type{var"#s826"} where var"#s826"<:UpperTriangular, !Matched::Union{UnitUpperTriangular, UpperTriangular}) at /buildworker/worker/package_linux64/build/usr/share/julia/stdlib/v1.5/LinearAlgebra/src/special.jl:61
convert(::Type{var"#s826"} where var"#s826"<:UpperTriangular, !Matched::Union{LinearAlgebra.AbstractTriangular, Bidiagonal, Diagonal}) at /buildworker/worker/package_linux64/build/usr/share/julia/stdlib/v1.5/LinearAlgebra/src/special.jl:65
convert(::Type{T}, !Matched::T) where T<:AbstractArray at abstractarray.jl:14
...

Stacktrace:
[1] setproperty!(::Tracker.Tracked{UpperTriangular{Float64,Array{Float64,2}}}, ::Symbol, ::Array{Float64,2}) at ./Base.jl:34
[2] back(::Tracker.Tracked{UpperTriangular{Float64,Array{Float64,2}}}, ::Array{Float64,2}, ::Bool) at /home/yiyuezhuo/.julia/packages/Tracker/AMi2h/src/back.jl:53
[3] #13 at /home/yiyuezhuo/.julia/packages/Tracker/AMi2h/src/back.jl:38 [inlined]
[4] foreach at ./abstractarray.jl:2010 [inlined]
[5] back_(::Tracker.Call{Tracker.var"#521#522"{TrackedArray{…,UpperTriangular{Float64,Array{Float64,2}}},TrackedArray{…,UpperTriangular{Float64,Array{Float64,2}}}},Tuple{Tracker.Tracked{UpperTriangular{Float64,Array{Float64,2}}},Tracker.Tracked{UpperTriangular{Float64,Array{Float64,2}}}}}, ::Array{Float64,2}, ::Bool) at /home/yiyuezhuo/.julia/packages/Tracker/AMi2h/src/back.jl:38
[6] back(::Tracker.Tracked{UpperTriangular{Float64,Array{Float64,2}}}, ::Array{Float64,2}, ::Bool) at /home/yiyuezhuo/.julia/packages/Tracker/AMi2h/src/back.jl:58
[7] #13 at /home/yiyuezhuo/.julia/packages/Tracker/AMi2h/src/back.jl:38 [inlined]
[8] foreach at ./abstractarray.jl:2010 [inlined]
[9] back_(::Tracker.Call{Tracker.var"#494#495"{TrackedArray{…,UpperTriangular{Float64,Array{Float64,2}}}},Tuple{Tracker.Tracked{UpperTriangular{Float64,Array{Float64,2}}}}}, ::Float64, ::Bool) at /home/yiyuezhuo/.julia/packages/Tracker/AMi2h/src/back.jl:38
[10] back(::Tracker.Tracked{Float64}, ::Int64, ::Bool) at /home/yiyuezhuo/.julia/packages/Tracker/AMi2h/src/back.jl:58
[11] #back!#15 at /home/yiyuezhuo/.julia/packages/Tracker/AMi2h/src/back.jl:77 [inlined]
[12] #back!#32 at /home/yiyuezhuo/.julia/packages/Tracker/AMi2h/src/lib/real.jl:16 [inlined]
[13] back!(::Tracker.TrackedReal{Float64}) at /home/yiyuezhuo/.julia/packages/Tracker/AMi2h/src/lib/real.jl:14
[14] gradient_(::Function, ::Array{Float64,2}) at /home/yiyuezhuo/.julia/packages/Tracker/AMi2h/src/back.jl:4
[15] #gradient#24 at /home/yiyuezhuo/.julia/packages/Tracker/AMi2h/src/back.jl:164 [inlined]
[16] gradient(::Function, ::Array{Float64,2}) at /home/yiyuezhuo/.julia/packages/Tracker/AMi2h/src/back.jl:164
[17] top-level scope at In[43]:1
[18] include_string(::Function, ::Module, ::String, ::String) at ./loading.jl:1091
```

Product of two Triangular matrix (result is dense matrix):

```julia
function g2(x)
y = UpperTriangular(x)
sum(y' * y)
end

Tracker.gradient(g2, X)
```

throw:

```
MethodError: Cannot `convert` an object of type Base.ReshapedArray{Float64,2,Adjoint{Float64,Array{Float64,2}},Tuple{Base.MultiplicativeInverses.SignedMultiplicativeInverse{Int64}}} to an object of type UpperTriangular{Float64,Array{Float64,2}}
Closest candidates are:
convert(::Type{var"#s826"} where var"#s826"<:UpperTriangular, !Matched::Union{UnitUpperTriangular, UpperTriangular}) at /buildworker/worker/package_linux64/build/usr/share/julia/stdlib/v1.5/LinearAlgebra/src/special.jl:61
convert(::Type{var"#s826"} where var"#s826"<:UpperTriangular, !Matched::Union{LinearAlgebra.AbstractTriangular, Bidiagonal, Diagonal}) at /buildworker/worker/package_linux64/build/usr/share/julia/stdlib/v1.5/LinearAlgebra/src/special.jl:65
convert(::Type{T}, !Matched::T) where T<:AbstractArray at abstractarray.jl:14
...

Stacktrace:
[1] setproperty!(::Tracker.Tracked{UpperTriangular{Float64,Array{Float64,2}}}, ::Symbol, ::Base.ReshapedArray{Float64,2,Adjoint{Float64,Array{Float64,2}},Tuple{Base.MultiplicativeInverses.SignedMultiplicativeInverse{Int64}}}) at ./Base.jl:34
[2] back(::Tracker.Tracked{UpperTriangular{Float64,Array{Float64,2}}}, ::Base.ReshapedArray{Float64,2,Adjoint{Float64,Array{Float64,2}},Tuple{Base.MultiplicativeInverses.SignedMultiplicativeInverse{Int64}}}, ::Bool) at /home/yiyuezhuo/.julia/packages/Tracker/AMi2h/src/back.jl:53
[3] #13 at /home/yiyuezhuo/.julia/packages/Tracker/AMi2h/src/back.jl:38 [inlined]
[4] foreach at ./abstractarray.jl:2010 [inlined]
[5] back_(::Tracker.Call{Tracker.var"#392#393"{TrackedArray{…,UpperTriangular{Float64,Array{Float64,2}}}},Tuple{Tracker.Tracked{UpperTriangular{Float64,Array{Float64,2}}}}}, ::Array{Float64,2}, ::Bool) at /home/yiyuezhuo/.julia/packages/Tracker/AMi2h/src/back.jl:38
[6] back(::Tracker.Tracked{Adjoint{Float64,UpperTriangular{Float64,Array{Float64,2}}}}, ::Array{Float64,2}, ::Bool) at /home/yiyuezhuo/.julia/packages/Tracker/AMi2h/src/back.jl:58
[7] #13 at /home/yiyuezhuo/.julia/packages/Tracker/AMi2h/src/back.jl:38 [inlined]
[8] foreach at ./abstractarray.jl:2010 [inlined]
[9] back_(::Tracker.Call{Tracker.var"#521#522"{TrackedArray{…,Adjoint{Float64,UpperTriangular{Float64,Array{Float64,2}}}},TrackedArray{…,UpperTriangular{Float64,Array{Float64,2}}}},Tuple{Tracker.Tracked{Adjoint{Float64,UpperTriangular{Float64,Array{Float64,2}}}},Tracker.Tracked{UpperTriangular{Float64,Array{Float64,2}}}}}, ::Array{Float64,2}, ::Bool) at /home/yiyuezhuo/.julia/packages/Tracker/AMi2h/src/back.jl:38
[10] back(::Tracker.Tracked{Array{Float64,2}}, ::Array{Float64,2}, ::Bool) at /home/yiyuezhuo/.julia/packages/Tracker/AMi2h/src/back.jl:58
[11] #13 at /home/yiyuezhuo/.julia/packages/Tracker/AMi2h/src/back.jl:38 [inlined]
[12] foreach at ./abstractarray.jl:2010 [inlined]
[13] back_(::Tracker.Call{Tracker.var"#494#495"{TrackedArray{…,Array{Float64,2}}},Tuple{Tracker.Tracked{Array{Float64,2}}}}, ::Float64, ::Bool) at /home/yiyuezhuo/.julia/packages/Tracker/AMi2h/src/back.jl:38
[14] back(::Tracker.Tracked{Float64}, ::Int64, ::Bool) at /home/yiyuezhuo/.julia/packages/Tracker/AMi2h/src/back.jl:58
[15] #back!#15 at /home/yiyuezhuo/.julia/packages/Tracker/AMi2h/src/back.jl:77 [inlined]
[16] #back!#32 at /home/yiyuezhuo/.julia/packages/Tracker/AMi2h/src/lib/real.jl:16 [inlined]
[17] back!(::Tracker.TrackedReal{Float64}) at /home/yiyuezhuo/.julia/packages/Tracker/AMi2h/src/lib/real.jl:14
[18] gradient_(::Function, ::Array{Float64,2}) at /home/yiyuezhuo/.julia/packages/Tracker/AMi2h/src/back.jl:4
[19] #gradient#24 at /home/yiyuezhuo/.julia/packages/Tracker/AMi2h/src/back.jl:164 [inlined]
[20] gradient(::Function, ::Array{Float64,2}) at /home/yiyuezhuo/.julia/packages/Tracker/AMi2h/src/back.jl:164
[21] top-level scope at In[45]:6
[22] include_string(::Function, ::Module, ::String, ::String) at ./loading.jl:1091
```

While it seems that one can get gradient if just one argument is `Triangular` matrix:

```julia
function g3(x)
y = UpperTriangular(x)
sum(2*y' .+ 1)
end

Tracker.gradient(g3, X)
```

gives:
```
([2.0 2.0 2.0; 0.0 2.0 2.0; 0.0 0.0 2.0] (tracked),)
```

Start by reproducing g1 and g2 with Tracker.gradient, then read the back.jl paths shown in the stack traces and compare them with the working g3 case. Done means the two triangular-matrix product examples no longer raise the reported conversion errors and are covered by regression tests.

Written by the indexing model from the issue text.