Remove namespace lld { namespace coff { from COFF LLD cpp files

Instead, use `using namespace lld(::coff)`, and fully qualify the names
of free functions where they are defined in cpp files.

This effectively reverts d79c3be6 to follow the new style guide added
in 236fcbc2.

Reviewed By: MaskRay

Differential Revision: https://reviews.llvm.org/D74882