[gdb/symtab] Handle multiple .debug_info sections

When compiling dw2-multiple-debug-info.c using -gdwarf-5
-fdebug-types-section, we end with two .debug_info sections in the object
file:
...
$ g++ gdb.dwarf2/dw2-multiple-debug-info.c -c -g \
    -gdwarf-5 \
    -fdebug-types-section
$ readelf -WS dw2-multiple-debug-info.o | grep -v RELA | grep .debug_info
  [10] .debug_info  PROGBITS        0 000128 0000cd 00  GC  0   0  8
  [12] .debug_info  PROGBITS        0 0001f8 0000ad 00   C  0   0  8
...

One of them contains the CU for dw2-multiple-debug-info.c, the other contains
the TU for the type of variable a.

When trying to print the type of variable a, we get:
...
$ gdb -q -batch dw2-multiple-debug-info.o -ex "ptype a"
'a' has unknown type; cast it to its declared type
...
because the TU hasn't been read.

Fix this by adding support for reading multiple .debug_info sections, similar
to how that is done for multiple .debug_types sections, getting us instead:
...
$ gdb -q -batch dw2-multiple-debug-info.o -ex "ptype a"
type = class sp1::A {
  ...
}
...

Tested on x86_64-linux.

PR symtab/32223
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=32223

gdb/dwarf2/index-write.c

@@ -1548,6 +1548,9 @@ write_dwarf_index (dwarf2_per_bfd *per_bfd, const char *dir,
   if (table == nullptr)
     error (_("Cannot use an index to create the index"));
 
+  if (per_bfd->infos.size () > 1)
+    error (_("Cannot make an index when the file has multiple .debug_info"
+	     " sections"));
   if (per_bfd->types.size () > 1)
     error (_("Cannot make an index when the file has multiple .debug_types sections"));