Fix a GCC warning about incompatible member function pointer cast
Disable de-virtualization in union_cast, which effectively prevents crashes when optimizaton is enabled under GCC when mocking classes with inline virtual dtor.
Coverage remained the same at 99.923% when pulling 614c7851c6cc47b1149cab1a01f42aeb9705706e on hedayat:master into 814f28cd509ef2a8cdf7fe370bd9ec6d3b181dc7 on eranpeer:master.