[flang] Fix narrowing warning on macos

[tskeith]

Summary

With clang 11 on macos we were getting this warning:

flang/runtime/random.cpp:61:30: error: non-constant-expression cannot be narrowed from type 'unsigned long long' to 'runtime::GeneratedWord' (aka 'unsigned int') in initializer list [-Wc++11-narrowing]
          GeneratedWord word{(generator() - generator.min()) & rangeMask};
                             ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
flang/runtime/random.cpp:99:5: note: in instantiation of function template specialization 'runtime::Generate<double, 53>' requested here
    Generate<CppTypeFor<TypeCategory::Real, 8>, 53>(harvest);
    ^

Changing the type of rangeMask fixes it.

flang/runtime/random.cpp

inline void Generate(const Descriptor &harvest) {
   SubscriptValue at[maxRank];
   harvest.GetLowerBounds(at);
   {
     CriticalSection critical{lock};
     for (std::size_t j{0}; j < elements; ++j) {
       Int fraction{generator()};
       if constexpr (words > 1) {
         for (std::size_t k{1}; k < words; ++k) {
-          static constexpr Int rangeMask{(Int{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));
       harvest.IncrementSubscripts(at);