Fix compilation issues on 64-bit macOS with DMD host compiler >= 2.079

[kinke]

Should fix #2703.

[JohanEngelen]

This shows how bad the mangling interop has become :( we should point the DMD folks to it.

[JohanEngelen]

LGTM

[kinke]

This shows how bad the mangling interop has become :( we should point the DMD folks to it.

I told them ~1 month ago that size_t interop is huge. This is only the bare minimum to get LDC to link. I still count 82 occurrences of size_t in DMD's C++ headers included in LDC; not all of them would need to be replaced (struct/class fields), but most of them are function params.

[JohanEngelen]

This is the same fix This is fixing the same issue as dlang/dmd#8249, right?

[kinke]

Ah yeah apparently so (haven't really given that PR's changes a proper look before).