Fortran: different character lengths in array constructor [PR93289]

	PR fortran/93289

gcc/fortran/ChangeLog:

	* decl.cc (gfc_set_constant_character_len): Downgrade different
	string lengths in character array constructor to legacy extension.

gcc/testsuite/ChangeLog:

	* gfortran.dg/unlimited_polymorphic_1.f03: Pad element in character
	array constructor to correct length.
	* gfortran.dg/char_array_constructor_5.f90: New test.