[PECOFF] Implement the reader for the import library.

This is the first patch toward full DLL support. With this patch, lld can
read .lib file for a DLL.

Reviewers: Bigcheese

CC: llvm-commits

Differential Revision: http://llvm-reviews.chandlerc.com/D987

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@184101 91177308-0d34-0410-b5e6-96231b3b80d8