Move some of the common decorators to decorators.py.

This doesn't attempt to move every decorator.  The reason for
this is that it requires touching every single test file to import
decorators.py.  I would like to do this in a followup patch, but
in the interest of keeping the patches as bite-sized as possible,
I've only attempted to move the underlying common decorators first.
A few tests call these directly, so those tests are updated as part
of this patch.

llvm-svn: 259807