Disable testing of complex compound assignment operators for MSVC.

MSVC does not support specializing compound assignments for std::complex, since it already specializes them (contrary to the standard).

Trying to use one of these on device will currently lead to a duplicate definition error. This is still probably preferable to no error though. If we remove the definitions for MSVC, then it will compile, but the kernel will fail silently.

The only proper solution would be to define our own custom Complex type.