[flang] Fix "latest-clang" build warnings

A recent patch to fix warnings from a Windows build regarding
inconsistent "struct" vs "class" usage missed a few code sites
and now the "latest-clang" build bot is also failing.  This
patch should resolve those failing builds.

flang/include/flang/Evaluate/expression.h

@@ -882,10 +882,10 @@ FOR_EACH_INTRINSIC_KIND(extern template class ArrayConstructor, )
 #define INSTANTIATE_EXPRESSION_TEMPLATES \
   FOR_EACH_INTRINSIC_KIND(template class Expr, ) \
   FOR_EACH_CATEGORY_TYPE(template class Expr, ) \
-  FOR_EACH_INTEGER_KIND(template struct Relational, ) \
-  FOR_EACH_REAL_KIND(template struct Relational, ) \
-  FOR_EACH_CHARACTER_KIND(template struct Relational, ) \
-  template struct Relational<SomeType>; \
+  FOR_EACH_INTEGER_KIND(template class Relational, ) \
+  FOR_EACH_REAL_KIND(template class Relational, ) \
+  FOR_EACH_CHARACTER_KIND(template class Relational, ) \
+  template class Relational<SomeType>; \
   FOR_EACH_TYPE_AND_KIND(template class ExpressionBase, ) \
   FOR_EACH_INTRINSIC_KIND(template class ArrayConstructorValues, ) \
   FOR_EACH_INTRINSIC_KIND(template class ArrayConstructor, )