Fortran: Fix translatability of diagnostic strings

gcc/fortran/ChangeLog:

	* check.cc (is_c_interoperable): Use _(...) around to mark strings
	as translatable.
	* data.cc (gfc_assign_data_value): Move string literal to gfc_error
	to make it translatable.
	* resolve.cc (resolve_fl_variable, resolve_equivalence): Use G_(...)
	around string literals.
	* scanner.cc (skip_fixed_omp_sentinel): Replace '...' by %<...%>.
	* trans-openmp.cc (gfc_split_omp_clauses,
	gfc_trans_omp_declare_variant): Likewise.