[lld/mac] Use C++17 nested namespace syntax in most places

Some header files used

    namespace lld {
    namespace macho {
    // ...
    } // namespace macho
    std::string toString(const Type &t);
    } // namespace lld

In those files, I didn't use a nested namespace since it's not a big win there.

No behavior change.

Differential Revision: https://reviews.llvm.org/D131354