COFF: Fix short import lib import name type bitshift

As per the PE COFF spec (section 8.3, Import Name Type)
Offset: 18 Size 2 bits Name: Type
Offset: 20 Size 3 bits Name: Name Type

Offset: 20 added based on 18+2

Partially commited as rL279069

Differential Revision: https://reviews.llvm.org/D23540

llvm-svn: 283055