[flang][msvc] Avoid range-based for over initializer_list. NFC.

[Meinersbur]

Summary

Msvc crashes with "INTERNAL COMPILER ERROR" when iterating over an std::initializer_list in a constexpr constructor. Explicitly use the iterator instead.

This patch is part of the series to make flang compilable with MS Visual Studio.

[Meinersbur]

Bug was was already reported (but no yet fixed) at https://developercommunity.visualstudio.com/content/problem/1067774/ice-internal-compiler-error-on-constexpr-range-bas.html.

[tskeith]

flang/include/flang/Common/enum-set.h

40–41

To match the style of the flang project this should be:
for (auto it{enums.begin()}; it != enums.end(); ++it) {

[Meinersbur]

Apply @tskeith's style recommendation

flang/include/flang/Common/enum-set.h

public:
   // When the bitset fits in a word, use a custom local bitset class that is
   // more amenable to constexpr evaluation than the current std::bitset<>.
   using bitsetType =
       std::conditional_t<(BITS <= 64), common::BitSet<BITS>, std::bitset<BITS>>;
   using enumerationType = ENUM;
 
   constexpr EnumSet() {}
   constexpr EnumSet(const std::initializer_list<enumerationType> &enums) {
-    for (auto x : enums) {
-      set(x);
+    for (auto it{enums.begin()}; it != enums.end(); ++it) {
+      set(*it);
     }
   }
   constexpr EnumSet(const EnumSet &) = default;
   constexpr EnumSet(EnumSet &&) = default;
 
   constexpr EnumSet &operator=(const EnumSet &) = default;
   constexpr EnumSet &operator=(EnumSet &&) = default;