[mlir][EnumGen] Generate utility symbolizeEnum and stringifyEnum methods during EnumGen

Summary:
This revision adds generation of two utility methods during EnumGen:
```
llvm::Optional<EnumType> symbolizeEnum<EnumType>(llvm::StringRef)
<stringifyResult> stringifyEnum(EnumType);
```

This provides a generic interface for stringifying/symbolizing any enum that can be used in a template environment.

Differential Revision: https://reviews.llvm.org/D77937