Readdress r236990, use of static members on a non-static variable.

The TargetRegistry is just a namespace-like class, instantiated in one
place to use a range-based for loop. Instead, expose access to the
registry via a range-based 'targets()' function instead. This makes most
uses a bit awkward/more verbose - but eventually we should just add a
range-based find_if function which will streamline these functions. I'm
happy to mkae them a bit awkward in the interim as encouragement to
improve the algorithms in time.

llvm-svn: 237059