[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.
Patch by Jameson Nash <firstname.lastname@example.org> based on a patch
by David Majnemer.
Differential Revision: https://reviews.llvm.org/D21731