[lld] [lld-macho] Fix `Defined` size increase with `-mms-bitfields` (PR #107545)

[Martin Storsjö via llvm-commits]

https://github.com/mstorsjo approved this pull request.

LGTM, thanks!

The PR message is great, but it could also be good to further clarify that the failing static assert already is ifdeffed out for MSVC targets, but MinGW targets also use the same MS bitfield packing logic (for binary compatibility of C structs).

https://github.com/llvm/llvm-project/pull/107545