[flang] Add reduction semantics to fir.do_loop (#93934)
Derived from #92480. This PR introduces reduction semantics into loops
for DO CONCURRENT REDUCE. The `fir.do_loop` operation now invisibly has
the `operandSegmentsizes` attribute and takes variable-length reduction
operands with their operations given as `fir.reduce_attr`. For the sake
of compatibility, `fir.do_loop`'s builder has additional arguments at
the end. The `iter_args` operand should be placed in front of the
declaration of result types, so the new operand for reduction variables
(`reduce`) is put in the middle of arguments.
