[Object/COFF] Add import_directory_table_entry.

Summary: Add import_directory_table_entry to use for .idata section.

Reviewers: Bigcheese

CC: llvm-commits

Differential Revision: http://llvm-reviews.chandlerc.com/D1059

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@186379 91177308-0d34-0410-b5e6-96231b3b80d8