[InlineAsm] wrap Kind in enum class NFC

Should add some minor type safety to the use of this information, since
there's quite a bit of metadata being laundered through an `unsigned`.

I'm looking to potentially add more bitfields to that `unsigned`, but I
find InlineAsm's big ol' bag of enum values and usage of `unsigned`
confusing, type-unsafe, and un-ergonomic. These can probably be better
abstracted.

I think the lack of static_cast outside of InlineAsm indicates the prior
code smell fixed here.

Reviewed By: qcolombet

Differential Revision: https://reviews.llvm.org/D159242