This patch adds a new flag to lit --xfail-requires-pr which causes lit tests to fail if any XFAIL lines do not also reference a PR. To support this functionality this patch changes how XFAIL lines are parsed to allow one entry in the comma separated list to be a PR (starting with "http://llvm.org/PR").
By default this option is globally disabled in lit. It can be globally enabled by passing the flag, and it can be disabled in a test suite's lit.cfg or lit.site.cfg, using the option config.xFailRequirePR. Additionally extra PR prefixes can be specified globally using the --bug-tracker-prefix flag, or per-test suite by setting config.bugTrackerPrefixes.