Don't use DataRefImpl to implement ImportDirectoryEntryRef.

DataRefImpl (a union of two integers and a pointer) is not the ideal data type
to represent a reference to an import directory entity. We should just use the
pointer to the import table and an offset instead to simplify. No functionality
change.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@199349 91177308-0d34-0410-b5e6-96231b3b80d8