[Linker] Drop support for IR-level extended linking support (archives, etc.).

 - This code is dead, and the "right" way to get this support is to use the
   platform-specific linker-integrated LTO mechanisms, or the forthcoming LLVM
   linker.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@172749 91177308-0d34-0410-b5e6-96231b3b80d8