minor promotion issue with A[t|c]_ldiv_B for triangular A and non-strided B

[Sacha0]

Tracker for the triangular case of #14506 described in #14506 (comment). Briefly, where A is a triangular matrix and B is not a StridedVecOrMat, the element-type promotion for A[t|c]_ldiv_B sometimes fails. For example

m = 10
intmat = fill(1, m, m);
ltintmat = LowerTriangular(rand(1:10, m, m));
At_ldiv_B(ltintmat, sparse(intmat))

fails with an InexactError. See #14506 for more. Best!

[Sacha0]

Fixed.

[tkelman]

is it sufficiently tested to stay that way?

[Sacha0]

Probably not. I'll have a look.

[Sacha0]

I am not certain whether existing general tests cover this, so to be safe I added a specific test in #16968. Thanks!