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

Authored by MaskRay on Oct 7 2019, 1:31 AM.


[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