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

lib/CodeGen/AsmPrinter/DwarfDebug.cpp

@@ -1144,6 +1144,7 @@ bool DwarfDebug::addCurrentFnArgument(DbgVariable *Var, LexicalScope *Scope) {
   // arguments does the function have at source level.
   if (ArgNo > Size)
     CurrentFnArguments.resize(ArgNo * 2);
+  assert(!CurrentFnArguments[ArgNo - 1]);
   CurrentFnArguments[ArgNo - 1] = Var;
   return true;
 }