Support the designated initializer manglings

See itanium-cxx-abi/cxx-abi#27. Right now we only support the `<braced-expression> ::= <expression>` variant of the production so we don't have a separate type for `<braced-expression>`.  We should introduce one, with `Parse` and `Demangle` implementations, and support the other variants. This would be a good moderate complexity task for a contributor.