Enable debug fission for thinLTO linked via gold-plugin

Summary: This enables debug fission on implicit ThinLTO when linked with gold. It will put the .dwo files in a directory specified by user. 

Reviewers: tejohnson, pcc, dblaikie

Reviewed By: pcc

Subscribers: JDevlieghere, mehdi_amini, inglorion

Differential Revision: https://reviews.llvm.org/D44792

llvm-svn: 329988
diff --git a/llvm/lib/LTO/LTO.cpp b/llvm/lib/LTO/LTO.cpp
index e9ac314..e4dffbf 100644
--- a/llvm/lib/LTO/LTO.cpp
+++ b/llvm/lib/LTO/LTO.cpp
@@ -136,6 +136,7 @@
   AddString(Conf.AAPipeline);
   AddString(Conf.OverrideTriple);
   AddString(Conf.DefaultTriple);
+  AddString(Conf.DwoDir);
 
   // Include the hash for the current module
   auto ModHash = Index.getModuleHash(ModuleID);