[NFC][flang] Add FIXME for privatization of loop bounds in Bridge.cpp

There is post comment of adding TODO/FIXME for privatization of loop
bounds in D127137. D127137 fixes the bug in OpenMP firstprivate clause,
which should be refactored later according to the post comment. Add
FIXME for it.

Differential Revision: https://reviews.llvm.org/D130625