DWARF: add a language hook for scalar biased types

DWARF: add a language hook for scalar biased types

Front-ends like GNAT for Ada sometimes use biased encodings for integral
types.  This change creates a new language hook so that the bias
information can make it into the debugging information back-end and
introduces an experimental DWARF attribute to hold it.

gcc/ada/ChangeLog:

	* gcc-interface/misc.c (gnat_get_type_bias): New.
	(LANG_HOOKS_GET_TYPE_BIAS): Redefine macro to implement the
	get_type_bias language hook.

gcc/ChangeLog:

	* langhooks.h (struct lang_hooks_for_types): New get_bias_field.
	* langhooks-def.h (LANG_HOOKS_GET_TYPE_BIAS): New.
	(LANG_HOOKS_FOR_TYPES_INITIALIZER): Initialize the
	get_bias_field.
	* dwarf2out.c
	(base_type_die): In non-strict DWARF mode, invoke the
	get_type_bias language hook for INTEGER_TYPE nodes.  If it
	returns a bias, emit an attribute for it.
	(subrange_type_die): Change signature to handle bias.  If
	non-strict DWARF mode, emit an attribute for it, if one passed.
	(modified_type_die): For subrange types, invoke the
	get_type_bias langage hook and pass the bias to
	subrange_type_die.

From-SVN: r231767