Object: Teach llvm-ar to create symbol table for COFF short import files.

Object: Teach llvm-ar to create symbol table for COFF short import files.

COFF short import files are special kind of files that contains only
DLL-exported symbol names. That's different from object files because
it has no data except symbol names.

This change implements a SymbolicFile interface for the short import
files so that symbol names can be accessed through that interface.
llvm-ar is now able to read the file and create symbol table entries
for short import files.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@246276 91177308-0d34-0410-b5e6-96231b3b80d8