[mlir][OpFormatGen] Format enum attribute cases as keywords when possible

In the overwhelmingly common case, enum attribute case strings represent valid identifiers in MLIR syntax. This revision updates the format generator to format as a keyword in these cases, removing the need to wrap values in a string. The parser still retains the ability to parse the string form, but the printer will use the keyword form when applicable.

Differential Revision: https://reviews.llvm.org/D94575