Fix the lldb test bots

This addresses an accidental change in behavior from
41667a8b to get the bots back to green.
However, I think there's an issue with LLDB assuming it's valid to
enable support for keywords in language modes that don't support the
keyword (as other parts of Clang are not expecting to be able to do
that).

This should fix (and others):
https://lab.llvm.org/buildbot/#/builders/68/builds/38374