[clang] [analyzer] Move alpha checker EnumCastOutOfRange to optin (PR #67157)

[via cfe-commits]

DonatNagyE wrote:

@Xazax-hun This checker doesn't accept the use of bitwise enums (like the one in your example), so projects that use bitwise enums should not enable this checker. (Perhaps this limitation should be mentioned in the docs? @gamesh411)

It would be possible to write a more complex checker that recognizes and accepts enums that are used as flags; but I'm not sure that it's a good investment of developer-hours and I think that the current simple&stupid checker is already useful for some projects. 

https://github.com/llvm/llvm-project/pull/67157