Avoid type inference problem in chol which caused a 10x slowdown in stockcorr benchmark without breaking anything

@JeffBezanson I could solve the problem by calling another constructor. For solve reason `Triangular{eltype(C),typeof(C),uplo,false}(C)` worked better than `Triangular(C,uplo,false)` although the latter method is defined by
```
function Triangular{T}(A::AbstractMatrix{T}, uplo::Symbol, isunit::Bool=false)
    chksquare(A)
    Triangular{T,typeof(A),uplo,isunit}(A)
end
```
Is there something wrong with the latter definition that breaks type inference?