We probably want to keep the configuration for the default args (and the assignment to LLVM_LIT_ARGS).
Address @danalbert's comment about the default lit args.
As I read the implementation of add_lit_testsuite this will actually pass the args twice (LLVM_LIT_ARGS is passed implicitly, thank you, global scope!).
Rebase. Address @danalbert's comment.
I don't have cmake 3.2, so I can't check that the thing added in the rebase still works :(