[flang][msvc] Avoid a reinterpret_cast

The call to the binary->decimal formatter in real.cpp was cheating
by using a reinterpret_cast<> to extract its binary value.
Use a more principled and portable approach by extending the
API of evaluate::Integer<> to include ToUInt<>()/ToSInt<>()
member function templates that do the "right" thing.  Retain
ToUInt64()/ToSInt64() for compatibility.

Differential revision: https://reviews.llvm.org/D89435