[flang] Fix negative unit number hashing

[klausler]

Summary

Ensure that external unit number hashing produces a valid
index for a negative unit number, viz. a NEWUNIT=.

flang/runtime/unit-map.h

 // A simple hash table with forward-linked chains per bucket.
 
 #ifndef FORTRAN_RUNTIME_UNIT_MAP_H_
 #define FORTRAN_RUNTIME_UNIT_MAP_H_
 
 #include "lock.h"
 #include "memory.h"
 #include "unit.h"
+#include <cstdlib>
 
 namespace Fortran::runtime::io {
 
 class UnitMap {
 public:
   ExternalFileUnit *LookUp(int n) {
     CriticalSection critical{lock_};
     return Find(n);

 private:
   struct Chain {
     explicit Chain(int n) : unit{n} {}
     ExternalFileUnit unit;
     OwningPtr<Chain> next{nullptr};
   };
 
   static constexpr int buckets_{1031}; // must be prime
-  int Hash(int n) { return n % buckets_; }
+  int Hash(int n) { return std::abs(n) % buckets_; }
 
   ExternalFileUnit *Find(int n) {
     Chain *previous{nullptr};
     int hash{Hash(n)};
     for (Chain *p{bucket_[hash].get()}; p; previous = p, p = p->next.get()) {
       if (p->unit.unitNumber() == n) {
         if (previous) {
           // Move found unit to front of chain for quicker lookup next time