fortran: use_iso_fortran_env_module tweaks [PR118337]

fortran: use_iso_fortran_env_module tweaks [PR118337]

This patch adds a comment to explain why we initialize the non-constant
elts of symbol array separately and checking assert to verify that separate
initialization bumps the iterator for each macro.

2025-01-10  Jakub Jelinek  <jakub@redhat.com>

	PR fortran/118337
	* module.cc (use_iso_fortran_env_module): Add a comment explaining
	the optimization performed.  Add gcc_checking_assert that i was
	incremented for all the elements.  Formatting fix.