[OpAsmParser] Add a parseCommaSeparatedList helper and beef up Delimeter.

Lots of custom ops have hand-rolled comma-delimited parsing loops, as does
the MLIR parser itself.  Provides a standard interface for doing this that
is less error prone and less boilerplate.

While here, extend Delimiter to support <> and {} delimited sequences as
well (I have a use for <> in CIRCT specifically).

Differential Revision: https://reviews.llvm.org/D110122