[analyzer] Re-land 0aba69e "Add test directory for scan-build."
The tool is now looked for in the source directory rather than in the
install directory, which should exclude the problems with not being able
to find it.
The tests still aren't being run on Windows, but they hopefully will run
on other platforms that have shell, which hopefully also means Perl.
Differential Revision: https://reviews.llvm.org/D69781