Reapply r327151 (partially):

For our lldb customizations, instead of commenting out lines, use #ifdef
LLDB_ENABLE_ALL / #endif preprocess directives instead, so our diffs
against upstream only consist of added lines.

(Note that upstream has largely reshuffled the way optional lldb plugins
are handled, so we need a lot less of these #ifdefs. However, not all of
them can be dropped, unless we re-import several sources that we have
always skipped.)