C++: Support using enum declarations.

[sashabu]

Previously, using entries were classified into using-declarations and using-directives based on whether they refer to a namespace or not.

With using enum declarations, we can no longer use the target to distinguish using-declarations and using-enum-declarations. E.g. the following:

namespace foo {
enum class Bar { one, two };
}
using foo::Bar;  // Brings `Bar` into scope
using enum foo::Bar;  // Brings `one` and `two` into scope.
using foo::Bar::one; using foo::Bar::two;  // Brings `one` and `two` into scope with different syntax.

Therefore we add a kind column. For consistency and simplicity, using-directives are also migrated to a separate kind.

[jketema]

Two small questions, otherwise this looks fine to me.

[jketema]

Nothing in the database can refer to a UsingEntry, right?

[sashabu]

Fixed in af562f1. (Don't think we need to do anything on the upgrade side.)

[jketema]

I assume you did some basic testing to ensure this works as expected?

[sashabu]

Yes, I spot checked the version you reviewed by creating a database for the usings test with the before/after CLI and running the queries with after/before CLIs respectively. I'm doing the same for the using_container fix now.

[sashabu]

Just confirmed all looks good with the additional typo fix in 3defc8b.