[lldb] Fix the way we set up the lldb modules infrastructure.

D127284 introduced a new language option which is not benign from modules
perspective. Before this patch lldb would set up the compiler invocation and
later enable incremental processing. Post-D127284 this does not work because
the option causes a module hash mismatch for implicit modules.

In addition, D127284 enables parsing statements on the global scope if
incremental processing is on and thus `syntax_error_for_lldb_to_find` was
rightfully not recognized as a declaration and is considered a statement
which produces a slightly different diagnostic.

Thanks to Michael Buch for the help in understanding this issue. This patch
should appease the lldb bots.

More discussion available at: https://reviews.llvm.org/D127284