[CI] Add --succinct to lit args (#152335)
We currently log every single test that we run in premerge. This leads
to gigantic logs (200k+ lines on Linux) that can be difficult to parse
through. Having an indicator of progress is nice, especially for the
LLVM tests, but is not strictly necessary and not often used (I
imagine). Having a progress indicator from lit that works in CI cases is
on my TODO list.
For the rare cases where someone does need to see the list of tests that
run, the JUnit XML emitted by lit is available in the artifacts.
