Add experimental (minimal) DWARFv5 support.

This change depends on the new DWARFv5 constants mentioned in the
following draft: http://dwarfstd.org/doc/dwarf5.20141029.pdf

gcc/ChangeLog

	* doc/invoke.texi (-gdwarf-@{version}): Mention experimental DWARFv5.
	* opts.c (common_handle_option): Accept -gdwarf-5.
	* dwarf2out.c (is_cxx): Add DW_LANG_C_plus_plus_11 and
	DW_LANG_C_plus_plus_14.
	(lower_bound_default): Likewise. Plus DW_LANG_C11.
	(gen_compile_unit_die): Output DW_LANG_C_plus_plus_11,
	DW_LANG_C_plus_plus_14 or DW_LANG_C11.
	(output_compilation_unit_header): Output at most a DWARFv4 header.
	(output_skeleton_debug_sections): Likewise.
	(output_line_info): Likewise.
	(output_aranges): Document header version number.

gcc/testsuite/ChangeLog

	* gcc.dg/debug/dwarf2/lang-c11.c: New test.
	* gcc.dg/debug/dwarf2/lang-c11-d4-strict.c: Likewise.
	* g++.dg/debug/dwarf2/lang-cpp11.C: Likewise.
	* g++.dg/debug/dwarf2/lang-cpp14.C: Likewise.
	* g++.dg/debug/dwarf2/lang-cpp98.C: Likewise.

include/ChangeLog

	* dwarf2.h: Add DW_LANG_C_plus_plus_11, DW_LANG_C11 and
	DW_LANG_C_plus_plus_14.

From-SVN: r218077