calling `LinearAlgebra.eigen()`

gives warntype. Is it a bug?

```
import Statistics
import LinearAlgebra
function test(x::Matrix{Float64})
sigma = Statistics.cov(x)
eig = LinearAlgebra.eigen(sigma)
return eig
end
julia> @code_warntype test(randn(20, 5) )
Body::Union{Eigen{Complex{Float64},Complex{Float64},Array{Complex{Float64},2},Array{Complex{Float64},1}}, Eigen{Float64,Float64,Array{Float64,2},Array{Float64,1}}}
1 ─ %1 = Statistics.cov::Core.Compiler.Const(Statistics.cov, false)
│ %2 = invoke Statistics.:(#cov#37)(1::Int64, true::Bool, %1::Function, _2::Array{Float64,2})::Array{Float64,2}
│ %3 = LinearAlgebra.eigen::Core.Compiler.Const(LinearAlgebra.eigen, false)
│ %4 = invoke LinearAlgebra.:(#eigen#58)(true::Bool, true::Bool, %3::Function, %2::Array{Float64,2})::Union{Eigen{Complex{Float64},Complex{Float64},Array{Complex{Float64},2},Array{Complex{Float64},1}}, Eigen{Float64,Float64,Array{Float64,2},Array{Float64,1}}}
└── return %4
```