[flang] Control alignment of constant folded reals (llvm#149381)

When REAL types are constant folded, the underneath implementation uses
arrays of integers. Ensure that these arrays are properly aligned.

This matters when building flang with clang. In some cases, the
resulting code for flang compiler ended up using SSE2 aligned load
instructions for REAL(16) constant folding on x86_64, and these
instructions require that the values are loaded from the aligned
addresses.

Author: eugeneepshteyn

flang/include/flang/Evaluate/integer.h

@@ -74,6 +74,7 @@ class Integer {
   static_assert(std::is_unsigned_v<BigPart>);
   static_assert(CHAR_BIT * sizeof(BigPart) >= 2 * partBits);
   static constexpr bool littleEndian{IS_LITTLE_ENDIAN};
+  static constexpr int alignment{ALIGNMENT};
 
 private:
   static constexpr int maxPartBits{CHAR_BIT * sizeof(Part)};

flang/include/flang/Evaluate/real.h

@@ -490,7 +490,10 @@ template <typename WORD, int PREC> class Real {
       bool isNegative, int exponent, const Fraction &, Rounding, RoundingBits,
       bool multiply = false);
 
-  Word word_{}; // an Integer<>
+  // Require alignment, in case code generation on x86_64 decides that our
+  // Real object is suitable for SSE2 instructions and then gets surprised
+  // by unaligned address.
+  alignas(Word::alignment / 8) Word word_{}; // an Integer<>
 };
 
 extern template class Real<Integer<16>, 11>; // IEEE half format