In most cases this was already checked with `ClassLayout::isFixedLayout`. But for classes which are imported from another module and contain implementation-only C-imported stored properties, those properties don't show up in the class layout. Stack promoting such classes will reserve too less space on the stack which leads to all kind of memory corruption problems. The fix is to also check `ClassLayout::isFixedSize`, which returns false for such classes.
Fixes a miscompile
rdar://131067105
