[flang] add -flang-experimental-hlfir flag to flang-new

This flag instructs flang-new to use the new HLFIR lowering. It is
marked as experimental and not included in --help.

This was added to make it more convenient to test the performance of
code generated by the HLFIR lowering.

Extra diffs are from running clang-format on CLOptions.inc (which was
being forced by CI).

Differential Revision: https://reviews.llvm.org/D146278