Rewrite objc intrinsics to runtime methods in PreISelIntrinsicLowering instead of SDAG.

SelectionDAG currently changes these intrinsics to function calls, but that won't work
for other ISel's.  Also we want to eventually support nonlazybind and weak linkage coming
from the front-end which we can't do in SelectionDAG.

llvm-svn: 349552