Object: rename parameter from DLLName to ImportName

When I originally wrote this code, I neglected the fact that the import
library may be created for executables.  This name is not the name of
the DLL, but rather the name for the imported module.  It will be
embedded into the IAT/ILT reference.  Rename it to make it more obvious.
NFC.

llvm-svn: 308384