[PECOFF] Refactor _imp_ symbol generator.

We are going to have another type of jump table for the delay-load
import table. In order to prepare for that, we want to factor out
the function handling the jump table. No functionality change.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@219446 91177308-0d34-0410-b5e6-96231b3b80d8