[AutoDiff] Clean up parsing and printing. (#28376)
- Use general `Parser::isIdentifier(Token, StringRef)` function.
- Remove specialized `isWRTIdentifier`, `isJVPIdentifier`, `isVJPIdentifier`
functions from `Parser`.
- Clarify doc comments and parameter nullability for attribute printing code:
- `getDifferentiationParametersClauseString`
- `getTransposedParametersClauseString`
- Minor formatting and naming updates.
