[flang] Fix narrowing warning on macos

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.

Differential Revision: https://reviews.llvm.org/D100320