[Flang][OpenMP] Provide an error when the minus operator is used in a reduction

OpenMP 5.2 standard has deprecated the usage of the minus operation in
reductions. The minus operation also is an unpleasant feature with
varied interpretations.

The patch also changes the usage of the minus operator in some existing testcases.

Discussed in https://discourse.llvm.org/t/openmp-runtime-problem-with-subtraction-reduction/64404

Reviewed By: peixin

Differential Revision: https://reviews.llvm.org/D132060