Fix assignment operator issue for latest MSVC+NVCC.

Antonio Sánchezrequested to merge
cantonios/eigen:msvc_operator_mess into master

Details are scattered across #920 (closed), #1000 (closed), #1324 (closed), #2291 (closed).

Summary: some MSVC versions have a bug that requires omitting explicit operator= definitions (leads to duplicate definition errors), and some MSVC versions require adding explicit operator= definitions (otherwise implicitly deleted errors). This mess tries to cover all the cases encountered.

Fixes #2291 (closed).

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