Reapply "[CodeGen] Fix invalid DWARF info on Win64"

This reapplies rL289013 (reverted in rL289014) with the fixes identified
in D21731. Should hopefully pass the buildbots this time.

llvm-svn: 290809
diff --git a/llvm/docs/Extensions.rst b/llvm/docs/Extensions.rst
index 6c50abf..850c427 100644
--- a/llvm/docs/Extensions.rst
+++ b/llvm/docs/Extensions.rst
@@ -67,7 +67,7 @@
     .long 4
     .long 242
     .long 40
-    .secrel32 _function_name
+    .secrel32 _function_name + 0
     .secidx   _function_name
     ...