[lldb] Fix TestErrorMessages test on standalone builds by adding lldb-server substitution

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.

Reviewed By: labath

Differential Revision: https://reviews.llvm.org/D91155