Object/COFF: Fix possible truncation bug.

VA can be 64 bit, as the image base can be larger than 4GB, so we need to
handle 64 bit VAs properly.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@201803 91177308-0d34-0410-b5e6-96231b3b80d8