libiberty: Add get_DW_UT_name and update include/dwarf2.{def,h}

This adds a get_DW_UT_name function to dwarfnames using dwarf2.def
for use in binutils readelf to show the unit types in a DWARF5 header.

Also remove DW_CIE_VERSION which was already removed in binutils/gdb
and is not used in gcc.

include/ChangeLog:

	* dwarf2.def: Add DWARF5 Unit type header encoding macros
	DW_UT_FIRST, DW_UT and DW_UT_END.
	* dwarf2.h (enum dwarf_unit_type): Removed and define using
	DW_UT_FIRST, DW_UT and DW_UT_END macros.
	(DW_CIE_VERSION): Removed.
	(get_DW_UT_name): New function declaration.

libiberty/ChangeLog:

	* dwarfnames.c (get_DW_UT_name): Define using DW_UT_FIRST, DW_UT
	and DW_UT_END.