[cDAC] Set RejitFlags to use a known underlying type

[max-charlamb]

The previous implementation using an enum should be the size of a uint32_t but it isn't explicitly defined. To make the size of RejitFlags more clear, changing it to an enum class with an explicit uint32_t backing.

Will be used #109936 by the cDAC GetMethodDescData to fetch ReJIT information.

[dotnet-policy-service]

Tagging subscribers to this area: @tommcdon
See info in area-owners.md if you want to be subscribed.

[davmason]

LGTM modulo the comment about usage.

[davmason]

Could we do some C++ to make us not have to write out ILCodeVersion::RejitFlags:: every time? I think we could put using enum RejitFlags in the ILCodeVersion class definition so the usage is the same. It feels too long to type out all of it every time.

[max-charlamb]

I don't think this is possible to do for enum class's in C++14. It looks like using enum is only available in C++20 and higher.

I see some approaches online manually dumping each enum value, but that feels worse than referencing them directly. https://stackoverflow.com/a/48932002

[davidwrighton]

It is a bit unfortunate. Sometimes in the past I've simply elevated the enum to no longer be contained within the class when I do this conversion. Then I can just use RejitFlags::BLAH wherever it needs to be used. You still have to touch all of the places where the enum is touched, but the major reason to put an enum within ILCodeVersion was probably to give its members a namespace to live in, and enum class does that all by itself.

[max-charlamb]

I pulled the enum out of ILCodeVersion. Now it is referenced with RejitState::<enum_value> instead of ILCodeVersion::<enum_value>.

[davidwrighton]

I've made another comment about how I might have pulled the enum out of the ILCodeVersion class to avoid the ILCodeVersion::RejitState prefix on everything, but it is not critical to me.