nonsquare (underdetermined) LQ solves

[stevengj]

Fixes #34348.

Seems to be about 2x faster than pivoted QR for a large underdetermined solve, with similar accuracy (assuming full row rank):

julia> A = rand(1000,1500); b = rand(size(A,1));

julia> @btime lq($A) \ $b;
  101.955 ms (14 allocations: 19.37 MiB)

julia> @btime qr($A, Val{true}()) \ $b;
  250.584 ms (8046 allocations: 31.98 MiB)

julia> x = lq(A) \ b; norm(x - qr(A, Val{true}()) \ b) / norm(x)
3.5357944054772334e-15

[dkarrasch]

One of us is getting over- and under-determined wrong, perhaps worth a second thought. The other issue I couldn't see resolved is ldiv!, which promises solve in-place of B, but must necessarily widen/stretch B to fit the longer X in A*X = B. It's not easy to track the changes in the test, but I couldn't find tests for ldiv!.

[stevengj]

For non-square problems with more cols n than rows m, ldiv! only looks at the first m rows of its input (which should have n rows to store the output solution)

(As noted above, ldiv! is exercised by the tests because it is actually called by \.)

You’re right that I was mixing up over/under determined some of the time — it's fixed now.

[stevengj]

win32 CI failure seems to be an unrelated Error in testset SharedArrays

[stevengj]

win32 failure is now another unrelated glitch cp: cannot stat 'dist-extras/7z.*': No such file or directory.

Should be good to merge?