[pseudo] Share the underly payload when stripping comments for a token stream

`stripComments(cook(...))` is a common pattern being written.
Without this patch, this has a use-after-free issue (cook returns a temporary
TokenStream object which has its own payload, but the payload is not
shared with the one returned by stripComments).

Reviewed By: sammccall

Differential Revision: https://reviews.llvm.org/D125311