Reimplement dwarf_unit_type_name

I noticed that dwarf_unit_type_name is nearly identical to
get_DW_UT_name from libiberty; but rather than simply replacing it, it
seemed better to have it work like the other DWARF constant
stringification functions -- return a string showing unrecognized
numeric forms rather than nullptr.  (The previous code did include
numeric values for the recognized constants, but this seems to be not
that useful to me.)

2021-03-18  Tom Tromey  <tromey@adacore.com>

	* dwarf2/stringify.c (dwarf_unit_type_name): New function.  Use
	get_DW_UT_name.
	* dwarf2/stringify.h (dwarf_unit_type_name): Declare.
	* dwarf2/comp-unit.c (dwarf_unit_type_name): Remove.