Fix handling of implicit_pure by checking if non-pure procedures are called.

Fix handling of implicit_pure by checking if non-pure procedures are called.

Procedures are marked as implicit_pure if they fulfill the criteria of
pure procedures.  In this case, a procedure was not marked as not being
implicit_pure which called another procedure, which had not yet been
marked as not being implicit_impure.

Fixed by iterating over all procedures, setting callers of procedures
which are non-pure and non-implicit_pure as non-implicit_pure and
doing this until no more procedure has been changed.

gcc/fortran/ChangeLog:

2020-07-19  Thomas Koenig  <tkoenig@gcc.gnu.org>

	PR fortran/96018
	* frontend-passes.c (gfc_check_externals): Adjust formatting.
	(implicit_pure_call): New function.
	(implicit_pure_expr): New function.
	(gfc_fix_implicit_pure): New function.
	* gfortran.h (gfc_fix_implicit_pure): New prototype.
	* parse.c (translate_all_program_units): Call gfc_fix_implicit_pure.