Teach op parsers to use enum keyword parsing helpers

[seven-mile]

Similar to LLVM dialect, we have enum keyword parsing helpers like parseOptionalCIRKeyword and parseCIRKeyword. We should reuse them to implement existing enum parsing logic, e.g. function visibility and switch case kind.

Originally posted by @bcardosolopes in #760 (comment)

[chansuke]

Hello. I am new to Clang IR and am interested in working on this issue.
Could you please assign it to me? Thank you!