[CodeGen] Fix invalid DWARF info on Win64

The relocations for `DIEEntry::EmitValue` were wrong for Win64
(emitting FK_Data_4 instead of FK_SecRel_4). This corrects that
oversight so that the DWARF data is correct in Win64 COFF files.

Fixes PR15393.

Patch by Jameson Nash <jameson@juliacomputing.com> based on a patch
by David Majnemer.

Differential Revision: https://reviews.llvm.org/D21731

llvm-svn: 289013
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
     ...