[llvm] [llvm] Use conventional enum declarations (NFC) (PR #166318)

[Kazu Hirata via llvm-commits]

kazutakahirata wrote:

> would it make sense to just use `enum class` instead, because it's safer?

Yes, it's safer but I would have to update use each use site.


https://github.com/llvm/llvm-project/pull/166318