[flang] Cope with overflow in real MOD/MODULO

In folding and in the runtime library for real MOD/MODULO(A,P),
detect overflow from the division A/P and return a properly signed
zero result.  (When A/P overflows and both A and P are finite numbers
with nonzero P, the quotient would be a large integer when rounded to
the precision of the floating-point representation.)

Differential Revision: https://reviews.llvm.org/D129020