Conditionalize the use of DW_LANG_C_plus_plus_03 and DW_LANG_Rust

Conditionalize the use of DW_LANG_C_plus_plus_03 and DW_LANG_Rust

Older elfutils (pre-0.170) don't define these constants in dwarf.h so
don't use them in that case.

	* include/abg-ir.h (LANG_C_plus_plus_03): Add this new language
	enum to "enum translation_unit::language".
	* src/abg-dwarf-reader.cc (dwarf_language_to_tu_language): Do not
	use DW_LANG_Rust or DW_LANG_C_plus_plus_03 if these are not
	defined.
	(get_default_array_lower_bound): Handle the new
	translation_unit::LANG_C_plus_plus_03 enumerator.

Signed-off-by: Mark Wielaard <mark@klomp.org>
Signed-off-by: Dodji Seketeli <dodji@redhat.com>