Try to unbreak Win build differently after 97351982

Looks like the MS STL wants StringMapKeyIterator::operator*() to be const.
Return the result by copy instead of reference to do that.
Assigning to a hash map key iterator doesn't make sense anyways.

Also reverts 123f811f which is now hopefully no longer needed.

Differential Revision: https://reviews.llvm.org/D109167