Attempt to fix test on Windows.

I'm assuming that because sys::path::append defaults to native path
this fails. Since dsymutil is a Darwin tool, we can just hardcode the
UNIX path separator.

(cherry picked from commit 2d460f2)

 Conflicts:
	llvm/lib/DWARFLinkerParallel/DWARFLinkerCompileUnit.cpp

Author: adrian-prantl

llvm/lib/DWARFLinker/DWARFLinker.cpp

@@ -187,7 +187,7 @@ static SmallString<128> guessToolchainBaseDir(StringRef SysRoot) {
     return Result;
   Base = sys::path::parent_path(Base);
   Result = Base;
-  sys::path::append(Result, "Toolchains");
+  Result += "/Toolchains";
   return Result;
 }