Use default lower bound for vector types in debug info

Vector types are represented as array types with DW_AT_GNU_vector attribute
in the debug info and a range [0 .. TYPE_VECTOR_SUBPARTS - 1].  Now that's
obviously skewed toward the C family of languages, therefore the attached
patch changes the lower bound to the default for the language of the CU.

gcc/
	* dwarf2out.cc (gen_array_type_die): Use the default lower bound of
	the language for vector types.