[gdb/symtab] Dump qualified name of cooked_index_entry

When doing "maint print objfiles" for the exec of test-case
gdb.dwarf2/pr13961.exp, we get:
...
    [25] ((cooked_index_entry *) 0x37b25d0)
    name:       foo
    canonical:  foo
    DWARF tag:  DW_TAG_class_type
    flags:      0x0 []
    DIE offset: 0x2a
    parent:     ((cooked_index_entry *) 0)

    [26] ((cooked_index_entry *) 0x37b2630)
    name:       foo
    canonical:  foo
    DWARF tag:  DW_TAG_class_type
    flags:      0x0 []
    DIE offset: 0x25
    parent:     ((cooked_index_entry *) 0x37b25d0) [foo]
...

By following the parent links in the text, we can conclude that the qualified
name of DIE 0x25 is foo::foo (which is incorrect, that's PR symtab/30739).

But it's not evident, and also hard to verify in a test-case.

Add dumping of the qualified name, such that we have:
...
    [25] ((cooked_index_entry *) 0x333b5d0)
    name:       foo
    canonical:  foo
    qualified:  foo
    DWARF tag:  DW_TAG_class_type
    flags:      0x0 []
    DIE offset: 0x2a
    parent:     ((cooked_index_entry *) 0)

    [26] ((cooked_index_entry *) 0x333b630)
    name:       foo
    canonical:  foo
    qualified:  foo::foo
    DWARF tag:  DW_TAG_class_type
    flags:      0x0 []
    DIE offset: 0x25
    parent:     ((cooked_index_entry *) 0x333b5d0) [foo]
...

Tested on x86_64-linux.

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