llvm/CodeGen/CommandFlags.h: Prune doubleslash in #include.

While packaging 3.7 for Fedora, the debug info splitting
process fell over this, so fix it upstream seems like a good plan.

This should be put in the 3.7 branch as well.

Noticed by Dave Airlie <airlied@redhat.com>

llvm-svn: 247757