[DebugInfo][SIL] Introduce the 'implicit' attribute for debug variable

Debug variables that are marked 'implicit' on its `debug_value`
instruction mean that they were generated by compiler. Optimizers are
free to remove them (if it becomes a dead code, for instance) even in
-Onone. Since they are barely used by users and keeping them might lead
to incorrect IRGen results.

lib/SILOptimizer/Utils/InstOptUtils.cpp

@@ -222,7 +222,11 @@ bool swift::hasOnlyEndOfScopeOrDestroyUses(SILInstruction *inst) {
       // Include debug uses only in Onone mode.
       if (isDebugUser && inst->getFunction()->getEffectiveOptimizationMode() <=
                              OptimizationMode::NoOptimization)
-        return false;
+        if (auto DbgVarInst = DebugVarCarryingInst(user)) {
+          auto VarInfo = DbgVarInst.getVarInfo();
+          if (VarInfo && !VarInfo->Implicit)
+            return false;
+        }
     }
   }
   return true;