Fortran: diagnose strings of non-constant length in DATA statements [PR68569]

gcc/fortran/ChangeLog:

	PR fortran/68569
	* resolve.cc (check_data_variable): Do not accept strings with
	deferred length or non-constant length in a DATA statement.
	Reject also substrings of string variables of non-constant length.

gcc/testsuite/ChangeLog:

	PR fortran/68569
	* gfortran.dg/data_char_4.f90: Adjust expected diagnostic.
	* gfortran.dg/data_char_5.f90: Likewise.
	* gfortran.dg/data_char_6.f90: New test.