Fix MSVC atan2 test.

Antonio Sánchezrequested to merge
cantonios/eigen:atan2 into master

Add a correction when std::atan2 returns denorm_min, returning y/x on underflow according to the POSIX spec.

We potentially could add such a correction to numext::atan2, but I'm not sure it's necessary if the MSVC implementation does satisfy the C++ spec (but not POSIX).

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