Add a new lang-hook that provides the underlying base type of an
ENUMERAL_TYPE. The default implementation will just use type_for_size.
The implementation for C++ will use the ENUM_UNDERLYING_TYPE if it exists.
Use this enum_underlying_base_type lang-hook in dwarf2out.c to add a
DW_AT_type base type reference to a DW_TAG_enumeration.

gcc/
	* dwarf2out.c (gen_enumeration_type_die): Add DW_AT_type if DWARF
	version >= 3 or not strict DWARF.
	* langhooks.h (struct lang_hooks_for_types): Add
	enum_underlying_base_type.
	* langhooks.c (lhd_enum_underlying_base_type): New function.
	* gcc/langhooks.h (struct lang_hooks_for_types): Add
	enum_underlying_base_type.
	* langhooks-def.h (lhd_enum_underlying_base_type): New declaration.
	(LANG_HOOKS_ENUM_UNDERLYING_BASE_TYPE): New define.
	(LANG_HOOKS_FOR_TYPES_INITIALIZER): Add new lang hook.

gcc/cp/
	* cp-lang.c (cxx_enum_underlying_base_type): New function.
	(LANG_HOOKS_ENUM_UNDERLYING_BASE_TYPE): Define.

From-SVN: r210717