[lldb/Plugin] Use LLDB_PLUGIN_DECLARE to forward declare plugin initializers

Apparently Linux and Windows have the exact opposite behavior when it
comes to inline declarations of external functions. On Linux they're
considered to be part of the lldb_private namespace, while on Windows
they're considered to be part of the top level namespace. Somehow on
macOS, it doesn't really matter and both are fine...

At this point I don't know what to do, so I'm just adding the
LLDB_PLUGIN_DECLARE macros again as originally proposed in D74245.