[Native] Fix fallback atoms when writing to Native file.

There is no test as currently there is no call to the NativeWriter.

Future commits will add calls to the NativeWriter.

llvm-svn: 192944
diff --git a/lld/lib/ReaderWriter/Native/WriterNative.cpp b/lld/lib/ReaderWriter/Native/WriterNative.cpp
index 9f42b89..c0bc28d 100644
--- a/lld/lib/ReaderWriter/Native/WriterNative.cpp
+++ b/lld/lib/ReaderWriter/Native/WriterNative.cpp
@@ -153,6 +153,9 @@
     NativeUndefinedAtomIvarsV1 ivar;
     ivar.nameOffset = getNameOffset(atom);
     ivar.flags = (atom.canBeNull() & 0x03);
+    ivar.fallbackNameOffset = 0;
+    if (atom.fallback())
+      ivar.fallbackNameOffset = getNameOffset(*atom.fallback());
     _undefinedAtomIvars.push_back(ivar);
   }