DebugInfo: Recommit (reverted in r215217, originally committed in r215157) the assertion that no argument variable is overwritten by subsequent argument variables.

This turned up a bug in clang where arguments were emitted with
duplicate argument numbers (see r215227).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@215228 91177308-0d34-0410-b5e6-96231b3b80d8