Add a trivial testing infrastructure that could allow writing regression tests for the scan-build tool in case we ever try to improve it.
It's a "before i forget" kind of thing - i played with it a little bit but didn't go far, but then @aaron.ballman pointed out that he could have made some tests for D55665 if the infrastructure was in place.
An obvious next step is to figure out how to test scan-build output, which is annoying because it auto-creates a non-deterministic sub-directory and then produces random file names. I guess we could do wildcards, but we could also make a deterministic output mode; we already have a mode for deterministic file names, so only the date in the directory name is a problem.
Another nice thing to try here is to make lit automatically run the same tests for scan-build-py as well. I expect the %scan_build substitution to allow such tricks, similarly to how we run all Analyzer tests under the Z3 solver if it's enabled.