[gdb/symtab] Fix parent of enumerator

[gdb/symtab] Fix parent of enumerator

As mentioned in commit 489b82720f5 ('[gdb/symtab] Revert "Change handling of
DW_TAG_enumeration_type in DWARF scanner"'), when doing "maint print objfiles" in
test-case gdb.dwarf2/enum-type.exp, for val1 we get an entry without parent:
...
    [27] ((cooked_index_entry *) 0x7fbbb4002ef0)
    name:       val1
    canonical:  val1
    qualified:  val1
    DWARF tag:  DW_TAG_enumerator
    flags:      0x0 []
    DIE offset: 0x124
    parent:     ((cooked_index_entry *) 0)
...

This happens here in cooked_indexer::index_dies:
...
	      info_ptr = recurse (reader, info_ptr,
				  is_enum_class ? this_entry : parent_entry,
				  fully);
...
when we're passing down a nullptr parent_entry, while the parent of this_entry
is deferred.

Fix this in cooked_indexer::index_dies by passing down a deffered parent
instead, such that we get:
...
    [27] ((cooked_index_entry *) 0x7ff0e4002ef0)^M
    name:       val1^M
    canonical:  val1^M
    qualified:  ns::val1^M
    DWARF tag:  DW_TAG_enumerator^M
    flags:      0x0 []^M
    DIE offset: 0x124^M
    parent:     ((cooked_index_entry *) 0x7ff0e4002f20) [ns]^M
...

Tested on x86_64-linux.

Approved-By: Tom Tromey <tom@tromey.com>