[MSVC2015] dllexport for defaulted special class members

Clang doesn't dllexport defaulted special member function defaulted
inside class but does it if they defaulted outside class. MSVC doesn't
make any distinction where they were defaulted. Also MSVC 2013 and 2015
export different set of members. MSVC2015 doesn't emit trivial defaulted
x-tors but does emit copy assign operator.

Differential revision: http://reviews.llvm.org/D20422

llvm-svn: 270535