[lit] Print exit code in for unresolved (lldb)tests.

A test is marked unresolved when we're unable to find PASSED or FAILED
in the dotest output. Usually this is because we crashed and when that
happens the exit code can give a clue as to why. This patch adds the
exit code to the lit output to make it easier to investigate those
issues.

Differential revision: https://reviews.llvm.org/D66975

llvm-svn: 370413