MSVC++ interop: Missing special cases for struct PODness

[kinke]

MS(VC++) is weird; our PODness logic is incomplete - so far, only the special case that any constructor makes a struct a non-POD is handled:

ldc/gen/abi/abi.cpp

Lines 119 to 125 in b996d14

	bool TargetABI::isPOD(Type *t, bool excludeStructsWithCtor) {
	t = t->baseElemOf();
	if (t->ty != TY::Tstruct)
	return true;
	StructDeclaration *sd = static_cast<TypeStruct *>(t)->sym;
	return dmd::isPOD(sd) && !(excludeStructsWithCtor && sd->ctor);
	}

Non-public data fields seem to make a struct a non-POD too: #5113 (comment). Not sure if Microsoft had any more great ideas for special cases.

I bet DMD doesn't handle this either. Extending the upstream runnable_cxx/{cppa.d,extra-files/cppb.cpp} tests with the special-struct cases would be good, using a small struct <= 8 bytes which would otherwise be passed/returned in a register.