Don't inherit dllimport to inline move assignment operators

Current MSVC versions don't have move assignment operators, so we
can't rely on them being available in the dll. If we have the
definition, we can just use that directly. This breaks pointer
equality, but should work fine otherwise.

When there is an MSVC version that supports move assignment,
we can key this off the -fmsc-ver option.

http://reviews.llvm.org/D4105