[llvm] [lit] Add --report-failures-only option for lit test reports (PR #115439)

[Greg Bedwell via llvm-commits]

gregbedwell wrote:

Just for completeness, here was my justification for this change from the original review I posted:

> 
> This option is designed to save on disk space in cases where a CI system
> is running dozens of builds each day and is using the information in the
> XUnit report to display information about the tests. Due to the sheer
> number of tests in the LLVM repo each XUnit report is multiple MB in
> size and when multiplied by large numbers of builds these reports can
> consume swathes of disk space when each report is archived by CI unless
> an aggressive expiration strategy is in place, meaning that historical
> data about failures is lost.
> 
> In general, the most interesting results are those were a test has
> failed (UNRESOLVED/TIMEOUT/FAIL/XPASS) so this new option limits the
> report to only include these tests. This comes at the expense of
> continuity of test data (a new failure will show as a new test for
> example, and no duration data will be recorded for passing tests), but
> for some users this may be a trade-off worth making.

https://github.com/llvm/llvm-project/pull/115439