[CUDA] Rename check-prefixes in cuda-options.cu and cuda-unused-arg-warning.cu.

Summary:
Rename the args to be more human-readable.  Among other things, this
lets us get rid of a bunch of comments (e.g. "ensure we don't run the
linker"), greatly shortening these tests.

Also apply consistent formatting and fix some English nits while we're
at it.

Reviewers: tra

Differential Revision: http://reviews.llvm.org/D15975

llvm-svn: 257557