Append clang system include path for offloading tool chains.

[sfantao]

Summary

This patch adds clang system include path when offloading tool chains, e.g. CUDA, are used in the current compilation.

This fixes an issue detected by @rsmith in response to r275645.

[sfantao]

• Add missing context for review.

[tra]

LGTM.

lib/Driver/Tools.cpp

   // TODO: Add support for other programming models here.
 }
 
+/// Add the C include args of other offloading toolchains. If this is a host
+/// job, the device toolchains are added. If this is a device job, the host
+/// toolchains will be added.
+static void addExtraOffloadClangSystemIncludeArgs(Compilation &C,
+                                                  const JobAction &JA,
+                                                  const ArgList &Args,
+                                                  ArgStringList &CmdArgs) {
+
+  if (JA.isHostOffloading(Action::OFK_Cuda))
+    C.getSingleOffloadToolChain<Action::OFK_Cuda>()->AddClangSystemIncludeArgs(
+        Args, CmdArgs);
+  else if (JA.isDeviceOffloading(Action::OFK_Cuda))
+    C.getSingleOffloadToolChain<Action::OFK_Host>()->AddClangSystemIncludeArgs(
+        Args, CmdArgs);
+
+  // TODO: Add support for other programming models here.
+}
+
 /// Add the include args that are specific of each offloading programming model.
 static void addExtraOffloadSpecificIncludeArgs(Compilation &C,
                                                const JobAction &JA,

   // Add system include arguments for all targets but IAMCU.
   if (!IsIAMCU) {
     getToolChain().AddClangSystemIncludeArgs(Args, CmdArgs);
-    addExtraOffloadCXXStdlibIncludeArgs(C, JA, Args, CmdArgs);
+    addExtraOffloadClangSystemIncludeArgs(C, JA, Args, CmdArgs);
   } else {
     // For IAMCU add special include arguments.
     getToolChain().AddIAMCUIncludeArgs(Args, CmdArgs);