[lit] Do a better job at parsing unsupported tests.

When all the tests run by dotest are unsupported, it still reports
RESULT: PASSED which we translate to success for lit. We can better
report the status as unsupported when we see that there are unsupported
tests but no passing tests. This will not affect the situation where
there are failures or unexpected passes, because those report a non-zero
exit code.

Differential revision: https://reviews.llvm.org/D68039

llvm-svn: 372914