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.
Reapplies 283390 with a forgotten testcase.
llvm-svn: 283400
Adrian Prantl