Fortran: bogus warnings with REPEAT intrinsic and -Wconversion-extra [PR96724]

gcc/fortran/ChangeLog:

	PR fortran/96724
	* iresolve.cc (gfc_resolve_repeat): Force conversion to
	gfc_charlen_int_kind before call to gfc_multiply.

gcc/testsuite/ChangeLog:

	PR fortran/96724
	* gfortran.dg/repeat_8.f90: New test.

Co-authored-by: José Rui Faustino de Sousa <jrfsousa@gmail.com>