[ELF2] Add --[no-]whole-archive command line switches

Summary:
If --whole-archive is used, all symbols from the following archives are added to the output. --no-whole-archive restores default behavior. These switches can be used multiple times.

NB. We have to keep an ArchiveFile instance within SymbolTable even if --whole-archive mode is active since it can be a thin archive which contains just names of external files. In that case actual memory buffers for the archive members will be stored within the File member of ArchiveFile class.

Reviewers: rafael, ruiu

Subscribers: grimar, llvm-commits

Projects: #lld

Differential Revision: http://reviews.llvm.org/D13286

llvm-svn: 249045