Set is_debug_types in allocate_signatured_type

Set is_debug_types in allocate_signatured_type

All callers of allocate_signatured_type set the is_debug_types flag on
the result -- in fact, they are required to, because this is the sign
that downcasting the object to signatured_type is safe.  This patch
moves this assignment into the allocation function.

2021-05-30  Tom Tromey  <tom@tromey.com>

	* dwarf2/read.c (dwarf2_per_bfd::allocate_signatured_type): Set
	is_debug_types.
	(create_signatured_type_table_from_index)
	(create_signatured_type_table_from_debug_names, add_type_unit)
	(read_comp_units_from_section): Update.