Fix invalid overflow check in flang

Statically checking for overflow with

    if constexpr (sizeof(std::size_t) <= sizeof(std::int64_t)) {
         return static_cast<std::int64_t>(length);
    }

Doesn't work if `sizeof(std::size_t) == sizeof(std::int64_t)` because std::size_t
is unsigned.

if `length == std::numeric_limits<size_t>` casting it to `int64_t` is going to overflow.

This code would be much simpler if returning a `uint64_t` instead of a signed
value...

Differential Revision: https://reviews.llvm.org/D122705