[gdb/symtab] Fix assert in read_addrmap_from_aranges

[gdb/symtab] Fix assert in read_addrmap_from_aranges

When loading the debug-names-duplicate-cu executable included in this
test-case, we run into:
...
(gdb) file debug-names-duplicate-cu^M
Reading symbols from debug-names-duplicate-cu...^M
src/gdb/dwarf2/read.c:2353: internal-error: read_addrmap_from_aranges: \
  Assertion `insertpair.second' failed.^M
...

This assert was added in recent commit 75337cbc147 ("[gdb/symtab] Fix
.debug_aranges duplicate offset warning").

The assert triggers because the CU table in the .debug_names section contains
a duplicate:
...
Version 5
Augmentation string: 47 44 42 00  ("GDB")
CU table:
[  0] 0x0
[  1] 0x0
...

Fix this by rejecting the .debug_names index:
...
(gdb) file debug-names-duplicate-cu^M
Reading symbols from debug-names-duplicate-cu...^M
warning: Section .debug_names has duplicate entry in CU table, \
  ignoring .debug_names.^M
...

Likewise for the case where the CU table is not sorted by increasing offset.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=29436