[flang][runtime] Ensure that 0. <= RANDOM_NUMBER() < 1.

[klausler]

Summary

It was possible for RANDOM_NUMBER() to return 1.0.

flang/runtime/random.cpp

inline void Generate(const Descriptor &harvest) {
   static constexpr std::size_t words{
       static_cast<std::size_t>(PREC + rangeBits - 1) / rangeBits};
   std::size_t elements{harvest.Elements()};
   SubscriptValue at[maxRank];
   harvest.GetLowerBounds(at);
   {
     CriticalSection critical{lock};
     for (std::size_t j{0}; j < elements; ++j) {
+      while (true) {
         Int fraction{generator()};
         if constexpr (words > 1) {
           for (std::size_t k{1}; k < words; ++k) {
-          static constexpr auto rangeMask{(GeneratedWord{1} << rangeBits) - 1};
+            static constexpr auto rangeMask{
+                (GeneratedWord{1} << rangeBits) - 1};
             GeneratedWord word{(generator() - generator.min()) & rangeMask};
             fraction = (fraction << rangeBits) | word;
           }
         }
         fraction >>= words * rangeBits - PREC;
-      *harvest.Element<REAL>(at) =
-          std::ldexp(static_cast<REAL>(fraction), -(PREC + 1));
+        REAL next{std::ldexp(static_cast<REAL>(fraction), -(PREC + 1))};
+        if (next >= 0.0 && next < 1.0) {
+          *harvest.Element<REAL>(at) = next;
+          break;
+        }
+      }
       harvest.IncrementSubscripts(at);
     }
   }
 }
 
 extern "C" {
 
 void RTNAME(RandomInit)(bool repeatable, bool /*image_distinct*/) {