Fix template step limit issue with clang

While working on PR 959, I instanciated a `common::TupleToVariant` with ~50+ types inside the tuple. Clang would then crash after 1hr compilation with message: "constexpr evaluation hit maximum step limit; possible infinite loop" After investigating, it turned out clang handles very badly the way `common::AreTypesDistinctHelper` was implemented. Its "number of steps" was exponential with the number of types. This fix makes this number quadratic which solves the issue.

Author: jeanPerier

include/flang/common/template.h

@@ -153,15 +153,12 @@ template<typename... Ts> struct VariantToTupleHelper<std::variant<Ts...>> {
 template<typename VARIANT>
 using VariantToTuple = typename VariantToTupleHelper<VARIANT>::type;
 
-template<typename A, typename B, typename... REST>
-struct AreTypesDistinctHelper {
+template<typename A, typename... REST> struct AreTypesDistinctHelper {
  static constexpr bool value() {
- if constexpr (std::is_same_v<A, B>) {
- return false;
- }
  if constexpr (sizeof...(REST) > 0) {
- return AreTypesDistinctHelper<A, REST...>::value() &&
- AreTypesDistinctHelper<B, REST...>::value();
+ // extra () for clang-format
+ return ((... && !std::is_same_v<A, REST>)) &&
+ AreTypesDistinctHelper<REST...>::value();
  }
  return true;
  }