Speed up A_ldiv_B! for QR factorizations

The major benefit is from eliminating calls to `cat`, however
I noticed that the type instability of `A[:Q]` was also a noteworthy
problem.

This essentially doubles the performance. It will get much faster yet
with faster array views (sub is now by far the major bottleneck).
Merge pull request #7888 from JuliaLang/teh/qrldiv

Speed up A_ldiv_B! for QR factorizations