[flang] prevent undefined behavior in character MAXLOC folding

When folding MAXLOC/MINLOC, the current element being compared was moved twice
in row in case it became the new extremum. With numeric and logical types, it
made no difference (std::move is a no-op for them), but for characters
where the string storage is actually moved, it caused the new extremum to
be set to the empty string, leading to wrong results.

Note: I could have left the first std::move relating to logical Findloc, but it
brings nothing and makes the code less auditable, so I also removed it.

Differential Revision: https://reviews.llvm.org/D122590