[FileCheck] Make invalid prefix diagnostics more precise

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