[MS] Don't statically initialize dllimport member function pointers

We were already applying the same rules to dllimport function pointers.
David Majnemer added that logic back in r211677 to fix PR20130.  We
failed to extend that logic to non-virtual member function pointers,
which are basically function pointers in a struct with some extra
offsets.

Fixes PR33570.

llvm-svn: 306137