Switch the representation of the location in instruction from

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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@100088 91177308-0d34-0410-b5e6-96231b3b80d8