NVPTX: Stop leaking memory by using a managed constant instead of a new Argument.

This is still an egregious hack since we don't have a nice interface for this
kind of thing but should help the valgrind leak check buildbot to become green.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@173267 91177308-0d34-0410-b5e6-96231b3b80d8