[flang][msvc] Avoid a reinterpret_cast

[klausler]

Summary

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.

Proposed as an alternative to https://reviews.llvm.org/D88798.

[Meinersbur]

Works fine with msvc.

flang/include/flang/Evaluate/integer.h

public:
   constexpr Ordering CompareSigned(const Integer &y) const {
     bool isNegative{IsNegative()};
     if (isNegative != y.IsNegative()) {
       return isNegative ? Ordering::Less : Ordering::Greater;
     }
     return CompareUnsigned(y);
   }
 
-  constexpr std::uint64_t ToUInt64() const {
-    std::uint64_t n{LEPart(0)};
-    int filled{partBits};
-    for (int j{1}; filled < 64 && j < parts; ++j, filled += partBits) {
-      n |= std::uint64_t{LEPart(j)} << filled;
+  template <typename UINT = std::uint64_t> constexpr UINT ToUInt() const {
+    UINT n{LEPart(0)};
+    std::size_t filled{partBits};
+    constexpr std::size_t maxBits{CHAR_BIT * sizeof n};
+    for (int j{1}; filled < maxBits && j < parts; ++j, filled += partBits) {
+      n |= UINT{LEPart(j)} << filled;
     }
     return n;
   }
 
-  constexpr std::int64_t ToInt64() const {
-    std::int64_t signExtended = ToUInt64();
-    if constexpr (bits < 64) {
-      signExtended |= -(signExtended >> (bits - 1)) << bits;
-    }
-    return signExtended;
+  template <typename SINT = std::int64_t, typename UINT = std::uint64_t>
+  constexpr SINT ToSInt() const {
+    SINT n = ToUInt<UINT>();
+    constexpr std::size_t maxBits{CHAR_BIT * sizeof n};
+    if constexpr (bits < maxBits) {
+      n |= -(n >> (bits - 1)) << bits;
+    }
+    return n;
+
+  constexpr std::uint64_t ToUInt64() const { return ToUInt<std::uint64_t>(); }
+
+  constexpr std::int64_t ToInt64() const {
+    return ToSInt<std::int64_t, std::uint64_t>();
   }
 
   // Ones'-complement (i.e., C's ~)
   constexpr Integer NOT() const {
     Integer result{nullptr};
     for (int j{0}; j < parts; ++j) {
       result.SetLEPart(j, ~LEPart(j));
     }

flang/lib/Evaluate/real.cpp

llvm::raw_ostream &Real<W, P>::AsFortran(
   } else if (IsInfinite()) {
     if (IsNegative()) {
       o << "(-1._" << kind << "/0.)";
     } else {
       o << "(1._" << kind << "/0.)";
     }
   } else {
     using B = decimal::BinaryFloatingPointNumber<P>;
-    const auto *value{reinterpret_cast<const B *>(this)};
-    char buffer[24000]; // accommodate real*16
+    B value{word_.template ToUInt<typename B::RawType>()};
+    char buffer[common::MaxDecimalConversionDigits(P) +
+        EXTRA_DECIMAL_CONVERSION_SPACE];
     decimal::DecimalConversionFlags flags{}; // default: exact representation
     if (minimal) {
       flags = decimal::Minimize;
     }
     auto result{decimal::ConvertToDecimal<P>(buffer, sizeof buffer, flags,
-        static_cast<int>(sizeof buffer), decimal::RoundNearest, *value)};
+        static_cast<int>(sizeof buffer), decimal::RoundNearest, value)};
     const char *p{result.str};
     if (DEREF(p) == '-' || *p == '+') {
       o << *p++;
     }
     int expo{result.decimalExponent};
     if (*p != '0') {
       --expo;
     }