[flang][msvc] Define implicit conversion from UnsignedInt128 to int64_t.

The custom implementation of UnsignedInt128 has an implicit conversion operator to unit64_t, but not int64_t. Considering that the former is already truncating, and C++ implicitly converts uint64_t to int64_t, UnsignedInt128  should also support an implicit conversion to int64_t. An analogous conversion would be from uint32_t to int16_t.

Without the conversion operator overload, the msvc emits the following error:
```
descriptor-io.h(44): error C2440: 'static_cast': cannot convert from 'A' to 'int64_t'
        with
        [
            A=Fortran::common::uint128_t
        ]
```

This patch is part of the series to make flang compilable with MS Visual Studio <http://lists.llvm.org/pipermail/flang-dev/2020-July/000448.html>.

Reviewed By: klausler

Differential Revision: https://reviews.llvm.org/D88509

Author: Meinersbur

flang/include/flang/Common/uint128.h

@@ -53,6 +53,7 @@ class UnsignedInt128 {
   constexpr bool operator!() const { return !low_ && !high_; }
   constexpr explicit operator bool() const { return low_ || high_; }
   constexpr explicit operator std::uint64_t() const { return low_; }
+  constexpr explicit operator std::int64_t() const { return low_; }
   constexpr explicit operator int() const { return static_cast<int>(low_); }
 
   constexpr std::uint64_t high() const { return high_; }