[MachO] Move type size asserts to source files. NFC

As discussed in https://reviews.llvm.org/D113809#3128636. It's a bit
unfortunate to move the asserts away from the structs whose sizes
they're checking, but it's a far better developer experience when one of
the asserts is violated, because you get a single error instead of every
single source file including the header erroring out.