It seems that TestErrorMessages.test is failing on the standalone + Xcode builds as lldb-server executable
can't be found by lit's default PATH search. I assume invoking lldb-server via a lit substitution
gets this working again as everything else is working, so that's what this patch is doing.
I had to add the lldb-server substitution as the test seems lldb-server specific and we don't want it to default
to debugserver on Darwin.
Using a substitution also seems in general like a good idea so that the commands lit is printing on failure
are using the full path to lldb-server and can be re-run in a terminal.