Fix NVCC+ICC issues.

NVCC does not understand `__forceinline`, so we need to use `inline`
when compiling for GPU.

ICC specializes `std::complex` operators for `float` and `double`
by default, which cannot be used on device and conflict with Eigen's
workaround in CUDA/Complex.h.  This can be prevented by defining
`_OVERRIDE_COMPLEX_SPECIALIZATION_` before including `<complex>`.
Added this define to the tests and to `Eigen/Core`, but this will
not work if the user includes `<complex>` before `<Eigen/Core>`.

ICC also seems to generate a duplicate `Map` symbol in
`PlainObjectBase`:
```
error: "Map" has already been declared in the current scope
  static ConstMapType Map(const Scalar *data)

```
I tracked this down to `friend class Eigen::Map`.  Putting the `friend`
statements at the bottom of the class seems to resolve this issue.

Fixes #2180

test/main.h

@@ -40,6 +40,8 @@
 // definitions.
 #include <limits>
 #include <algorithm>
+// Disable ICC's std::complex operator specializations so we can use our own.
+#define _OVERRIDE_COMPLEX_SPECIALIZATION_ 1
 #include <complex>
 #include <deque>
 #include <queue>