[DebugInfo] Use DbgEntityKind in DbgEntity interface (NFC)

It was being used occasionally already, and using it on the constructor
and getDbgEntityID has obvious type safety benefits.

Also use llvm_unreachable in the switch as usual, but since only these
two values are used in constructor calls I think it's still NFC.

Reviewed By: probinson

Differential Revision: https://reviews.llvm.org/D113862