This patch  is under review at https://reviews.llvm.org/D44788
It makes clang accept the option -glto-dwo-dir and pass
it to llvm to enable thinlto with debug fission.

--- a/tools/clang/docs/ClangCommandLineReference.rst
+++ b/tools/clang/docs/ClangCommandLineReference.rst
@@ -2587,6 +2587,8 @@ Debug information flags
 
 .. option:: -gstrict-dwarf, -gno-strict-dwarf
 
+.. option:: -glto-dwo-dir=<arg>
+
 .. option:: -gz
 
 DWARF debug sections compression type
diff --git a/include/clang/Driver/Options.td b/include/clang/Driver/Options.td
index 6925157d2e..83a4f5bc86 100644
--- a/tools/clang/include/clang/Driver/Options.td
+++ b/tools/clang/include/clang/Driver/Options.td
@@ -1700,6 +1700,7 @@ def gno_strict_dwarf : Flag<["-"], "gno-strict-dwarf">, Group<g_flags_Group>;
 def gcolumn_info : Flag<["-"], "gcolumn-info">, Group<g_flags_Group>, Flags<[CoreOption]>;
 def gno_column_info : Flag<["-"], "gno-column-info">, Group<g_flags_Group>, Flags<[CoreOption]>;
 def gsplit_dwarf : Flag<["-"], "gsplit-dwarf">, Group<g_flags_Group>;
+def glto_dwo_dir_EQ : Joined<["-"], "glto-dwo-dir=">, Group<g_flags_Group>;
 def ggnu_pubnames : Flag<["-"], "ggnu-pubnames">, Group<g_flags_Group>, Flags<[CC1Option]>;
 def gdwarf_aranges : Flag<["-"], "gdwarf-aranges">, Group<g_flags_Group>;
 def gmodules : Flag <["-"], "gmodules">, Group<gN_Group>,
diff --git a/lib/Driver/ToolChains/CommonArgs.cpp b/lib/Driver/ToolChains/CommonArgs.cpp
index 17ab82e97c..67992a655e 100644
--- a/tools/clang/lib/Driver/ToolChains/CommonArgs.cpp
+++ b/tools/clang/lib/Driver/ToolChains/CommonArgs.cpp
@@ -417,6 +417,19 @@
   if (IsThinLTO)
     CmdArgs.push_back("-plugin-opt=thinlto");
 
+  if (Arg *A = Args.getLastArg(options::OPT_glto_dwo_dir_EQ)) {
+    const char *Objcopy =
+        Args.MakeArgString(ToolChain.GetProgramPath(CLANG_DEFAULT_OBJCOPY));
+    CmdArgs.push_back(
+        Args.MakeArgString(Twine("-plugin-opt=objcopy=") + Objcopy));
+    StringRef Dwo_Dir = A->getValue();
+     SmallString<1024> DwoDir;
+     llvm::sys::path::native(Dwo_Dir, DwoDir);
+     llvm::sys::path::append(DwoDir, Twine(Output.getFilename()) + "_dwo");
+    CmdArgs.push_back(
+        Args.MakeArgString(Twine("-plugin-opt=dwo_dir=") + DwoDir));
+  }
+
   if (unsigned Parallelism = getLTOParallelism(Args, ToolChain.getDriver()))
     CmdArgs.push_back(
         Args.MakeArgString("-plugin-opt=jobs=" + Twine(Parallelism)));