[OpenMP] Fix translation of target args

[Hahnfeld]

Summary

ToolChain::TranslateArgs() returns nullptr if no changes are performed.
This would currently mean that OpenMPArgs are lost. Patch fixes this
by falling back to simply using OpenMPArgs in that case.

[gtbercea]

LGTM

lib/Driver/Compilation.cpp

Compilation::getArgsForToolChain(const ToolChain *TC, StringRef BoundArch,
   DerivedArgList *&Entry = TCArgs[{TC, BoundArch, DeviceOffloadKind}];
   if (!Entry) {
     SmallVector<Arg *, 4> AllocatedArgs;
     // Translate OpenMP toolchain arguments provided via the -Xopenmp-target flags.
     DerivedArgList *OpenMPArgs = TC->TranslateOpenMPTargetArgs(
         *TranslatedArgs, DeviceOffloadKind, AllocatedArgs);
     if (!OpenMPArgs) {
       Entry = TC->TranslateArgs(*TranslatedArgs, BoundArch, DeviceOffloadKind);
+      if (!Entry)
+        Entry = TranslatedArgs;
     } else {
       Entry = TC->TranslateArgs(*OpenMPArgs, BoundArch, DeviceOffloadKind);
+      if (!Entry)
+        Entry = OpenMPArgs;
+      else
         delete OpenMPArgs;
     }
 
-    if (!Entry)
-      Entry = TranslatedArgs;
-
     // Add allocated arguments to the final DAL.
     for (auto ArgPtr : AllocatedArgs) {
       Entry->AddSynthesizedArg(ArgPtr);
     }
   }
 
   return *Entry;
 }