[ELF] Wrap things in `namespace lld { namespace elf {`, NFC

This makes it clear `ELF/**/*.cpp` files define things in the `lld::elf`
namespace and simplifies `elf::foo` to `foo`.

Reviewed By: atanasyan, grimar, ruiu

Differential Revision: https://reviews.llvm.org/D68323

llvm-svn: 373885