Better instance construction for ldlt

Cannot be singular checked in some versions, so this is a nicer way to handle it.

Fixes SciML/OrdinaryDiffEq.jl#2041

Author: ChrisRackauckas

src/ArrayInterface.jl

@@ -546,13 +546,17 @@ Returns an instance of the LDLT factorization object with the correct type
 cheaply.
 """
 function ldlt_instance(A::Matrix{T}) where {T}  
-    return ldlt(SymTridiagonal(similar(A, 0, 0)))
+    return ldlt_instance(SymTridiagonal(similar(A, 0, 0)))
 end
 
 function ldlt_instance(A::SparseMatrixCSC)
     ldlt(sparse(similar(A, 1, 1)), check=false)
 end
 
+function ldlt_instance(A::SymTridiagonal{T,V}) where {T,V}
+    return LinearAlgebra.LDLt{T,SymTridiagonal{T,V}}(A)
+end
+
 """
 ldlt_instance(a::Number) -> a