Switch the representation of the location in instruction from

being a TrackingVH<MDNode> to a NewDebugLoc, shrinking 
sizeof(Instruction) a lot, and providing clients the ability
to deal with locations in terms of NewDebugLoc instead of
having to deal with Metadata.  This is still fully compatible
with all clients that *do* use MDNodes for everything of
course.

No functionality change.

llvm-svn: 100088