This will prove especially helpful after D79276, which introduces
comment prefixes. Specifically, identifying whether there's a
uniqueness violation will be helpful as prefixes will be required to
be unique across both check prefixes and comment prefixes.
Also, remove a related comment about `cl::list` that no longer seems
relevant now that FileCheck is also a library.
Reviewed By: jhenderson, thopre
Differential Revision: https://reviews.llvm.org/D79375
