[Driver][OpenMP] Update actions builder to create bundling action when necessary.

Summary:
In order to save the user from dealing with multiple output files (for host and device) while using separate compilation, a new action `OffloadBundlingAction` is used when the last phase is not linking. This action will then result in a job that uses the proposed bundling tool to create a single preprocessed/IR/ASM/Object file from multiple ones.

The job creation for the new action will be proposed in a separate patch.

Reviewers: echristo, tra, jlebar, ABataev, hfinkel

Subscribers: whchung, mehdi_amini, cfe-commits, Hahnfeld, andreybokhanko, arpith-jacob, carlo.bertolli, caomhin

Differential Revision: https://reviews.llvm.org/D21852

llvm-svn: 285323
diff --git a/clang/lib/Driver/ToolChain.cpp b/clang/lib/Driver/ToolChain.cpp
index 9023265..a82ebfe 100644
--- a/clang/lib/Driver/ToolChain.cpp
+++ b/clang/lib/Driver/ToolChain.cpp
@@ -263,6 +263,10 @@
   case Action::VerifyPCHJobClass:
   case Action::BackendJobClass:
     return getClang();
+
+  case Action::OffloadBundlingJobClass:
+    // FIXME: Add a tool for the bundling actions.
+    return nullptr;
   }
 
   llvm_unreachable("Invalid tool kind.");