Allow reference form for DW_AT_associated and DW_AT_allocated attributes

Currently, GDB rejects the (die) reference form while it accepts exprloc
form. It is allowed in DWARF standard. "Table 7.5: Attribute encodings"
in DWARF5 standard. Flang compiler assigns (die) reference to
DW_AT_associated and DW_AT_allocated for some cases.

gdb/ChangeLog

	* dwarf2/read.c (set_die_type): Removed conditions to restrict
	forms for DW_AT_associated and DW_AT_allocated attributes,
	which is already checked in function attr_to_dynamic_prop.