[COFF] Fix delay import directory iterator

Summary:
Take the Index into account in `getDelayImportTable`, otherwise we
always return the entry for the first delay DLL reference.

Reviewers: ruiu

Reviewed By: ruiu

Subscribers: hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D60081

llvm-svn: 357697