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

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 [[ http://lists.llvm.org/pipermail/flang-dev/2020-July/000448.html | make flang compilable with MS Visual Studio ]].

Reviewed By: isuruf

Differential Revision: https://reviews.llvm.org/D86425