IRGen: Write .dwo files when -split-dwarf-file is used together with -fthinlto-index.

Differential Revision: https://reviews.llvm.org/D47597

llvm-svn: 333677
diff --git a/clang/lib/CodeGen/BackendUtil.cpp b/clang/lib/CodeGen/BackendUtil.cpp
index f9239f6..f9855f0 100644
--- a/clang/lib/CodeGen/BackendUtil.cpp
+++ b/clang/lib/CodeGen/BackendUtil.cpp
@@ -1161,6 +1161,7 @@
   Conf.DebugPassManager = CGOpts.DebugPassManager;
   Conf.RemarksWithHotness = CGOpts.DiagnosticsWithHotness;
   Conf.RemarksFilename = CGOpts.OptRecordFile;
+  Conf.DwoPath = CGOpts.SplitDwarfFile;
   switch (Action) {
   case Backend_EmitNothing:
     Conf.PreCodeGenModuleHook = [](size_t Task, const Module &Mod) {