These member functions were decorated with `_LIBCPP_FUNC_VIS` when the
class is also decorated with external visibility. This breaks down when
building for PE/COFF, where the member function cannot be decorated if
it is within a decorated class. The class attribute will propagate to
the member. Remove the extraneous decoration.
git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@293454 91177308-0d34-0410-b5e6-96231b3b80d8