Revert "[OpenMP] Avoid VLAs for some reductions on array sections"

This breaks at least two buildbots:
http://lab.llvm.org:8011/builders/clang-cmake-x86_64-avx2-linux/builds/1175
http://lab.llvm.org:8011/builders/clang-atom-d525-fedora-rel/builds/10478

This reverts commit r316229 during local investigation.

llvm-svn: 316235