make SparseSolverBase and IterativeSolverBase move constructable

Alex_Mrequested to merge
Alex_rath3t/eigen:master into master

Hello everyone,

Thanks for this powerful library.

I witnessed an issue in my code where I wanted to move iterative solvers around.

Since they derive from SparseSolverBase they are not copyable.

This is fine, but I'm interested in the reason why this is the case.

Nevertheless, I propose the change to include move constructors, if C++11 is supported.

What does this implement/fix?

This fixes the issue that IterativeSolvers cannot be moved. I.e., it solves the issue outlined in https://godbolt.org/z/rM8M76bqW

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