Add results.xml for macOS lldb unittests

http://reviews.llvm.org/D21211

If gtest tests of lldb fail, there are no test results for Jenkins to
consume. This will make the results of the gtest failure available to
Jenkins.

This is to address the failure visible in:
http://lab.llvm.org:8080/green/job/lldb_build_test/19196/

llvm-svn: 272751