[UpdateTestChecks] Don't fail silently when conflicting CHECK lines means no checks are generated for some functions (#159321)

There is a warning that triggers if you (for instance) run
`update_llc_test_checks.py` on an input where _all_ functions have
conflicting check lines and so no checks are generated. However, there
are no warnings emitted at all for the case where some functions have
non-conflicting check lines but others don't. This is a source of
frustration because running update_llc_test_checks can result in all
check lines being removed for certain functions when such a conflict
exists with no warning, meaning we have to be extra vigilant inspecting
the diff. I've also personally wasted time tracking down the source of
the dropped lines assuming that update_test_checks would emit a warning
in such cases.

This change adds logic to emit warnings on a function-by-function basis
for any RUN that has conflicting prefixes meaning no output is
generated. This subsumes the previous warning for when _all_ functions
conflict.

0 files changed, 0 insertions, 0 deletions