Verifier: Reject any unknown named MD nodes in the llvm.dbg namespace.

This came out of a discussion in https://reviews.llvm.org/D25285.

There used to be various other llvm.dbg.* nodes, but we don't support
upgrading them and we want to reserve the namespace for future uses.

This also removes an entirely obsolete and bitrotted testcase for PR7662.

llvm-svn: 283390