[lldb] Parse the dotest output to determine the most appropriate result code

Currently we look for keywords in the dotest.py output to determine the
lit result code. This binary approach of a keyword being present works
for PASS and FAIL, where having at least one test pass or fail
respectively results in that exit code. Things are more complicated
for tests that neither passed or failed, but report a combination of
(un)expected failures, skips or unresolved tests.

This patch changes the logic to parse the number of tests with a
particular result from the dotest.py output. For tests that did not PASS
or FAIL, we now report the lit result code for the one that occurred the
most. For example, if we had a test with 3 skips and 4 expected
failures, we report the test as XFAIL.

We're still mapping multiple tests to one result code, so some loss of
information is inevitable.

Differential revision: https://reviews.llvm.org/D127258