Add a DAG test.

Doesn't do much yet...
Add a (very simple) Ninja backend test.

It doesn't do much, but it's end-to-end!
Add ast::Literal::str().

Literals should be able to produce string copies of themselves.
Add a proper "null" backend.
Trivial: fix a comment.
Don't produce backend output in most tests.
Leave behind intermediate results for inspection.
Use llvm-lit from LLVM 3.4.

This gives us CHECK-DAG, a very useful CHECK line.
Convert (top-level?) integer literals to DAG.