Switch autolinking metadata format over to actual linker options, e.g.,

  !0 = metadata !{metadata !"-lautolink"}
  !1 = metadata !{metadata !"-framework", metadata !"autolink_framework"}

referenced from llvm.module.linkoptions, e.g.,

  !llvm.module.linkoptions = !{!0, !1, !2, !3}

This conceptually moves the logic for figuring out the syntax the
linker will accept from LLVM into Clang. Moreover, it makes it easier
to support MSVC's

  #pragma comment(linker, "some option")

in the future, should anyone care to do so.