PECOFF: Reduce import table size.

Import Lookup Table in Import Directory Table has the same contents
as Hint/Name Table. Symbol names imported from DLLs are pointed by
both Import Directory Table and Hint/Name Table. We had duplicate
strings there.

This patch eliminates that duplication to make the table smaller.
This should reduce binary size by the sum of lengths of imported
symbols.

llvm-svn: 233128