[lld-macho][nfc] Remove `MachO::` prefix where possible

Previously, SyntheticSections.cpp did not have a top-level `using namespace
llvm::MachO` because it caused a naming conflict: `llvm::MachO::Symbol` would
collide with `lld::macho::Symbol`.

`MachO::Symbol` represents the symbols defined in InterfaceFiles (TBDs). By
moving the inclusion of InterfaceFile.h into our .cpp files, we can avoid this
name collision in other files where we are only dealing with LLD's own symbols.

Along the way, I removed all unnecessary "MachO::" prefixes in our code.

Cons of this approach: If TextAPI/MachO/Symbol.h gets included via some other
header file in the future, we could run into this collision again.

Alternative 1: Have either TextAPI/MachO or BinaryFormat/MachO.h use a different
namespace. Most of the benefit of `using namespace llvm::MachO` comes from being
able to use things in BinaryFormat/MachO.h conveniently; if TextAPI was under a
different (and fully-qualified) namespace like `llvm::tapi` that would solve our
problems. Cons: lots of files across llvm-project will need to be updated, and
folks who own the TextAPI code need to agree to the name change.

Alternative 2: Rename our Symbol to something like `LldSymbol`. I think this is
ugly.

Personally I think alternative #1 is ideal, but I'm not sure the effort to do it is
worthwhile, this diff's halfway solution seems good enough to me. Thoughts?

Reviewed By: #lld-macho, oontvoo, MaskRay

Differential Revision: https://reviews.llvm.org/D98149