[gdb] Support anonymous typedef generated by gcc -feliminate-dwarf2-dups

[gdb] Support anonymous typedef generated by gcc -feliminate-dwarf2-dups

Gcc supports an option -feliminate-dwarf2-dups (up until gcc-7, removed in
gcc-8).

When running tests with target board unix/-feliminate-dwarf2-dups, we run
into:
...
(gdb) PASS: gdb.ada/arraydim.exp: print m'length(3)
ptype global_3dim_for_gdb_testing^M
type = array (Unexpected type in ada_discrete_type_low_bound.^M
(gdb) FAIL: gdb.ada/arraydim.exp: ptype global_3dim_for_gdb_testing
...

The DWARF for the variable global_3dim_for_gdb_testing looks as follows:
...
 <0><824>: Abbrev Number: 1 (DW_TAG_compile_unit)
    <825>   DW_AT_name        : src/gdb/testsuite/gdb.ada/arraydim/inc.c
 <1><832>: Abbrev Number: 2 (DW_TAG_array_type)
    <833>   DW_AT_type        : <0x874>
 <2><837>: Abbrev Number: 3 (DW_TAG_subrange_type)
    <838>   DW_AT_type        : <0x84a>
    <83c>   DW_AT_upper_bound : 0
 <2><83d>: Abbrev Number: 3 (DW_TAG_subrange_type)
    <83e>   DW_AT_type        : <0x84a>
    <842>   DW_AT_upper_bound : 1
 <2><843>: Abbrev Number: 3 (DW_TAG_subrange_type)
    <844>   DW_AT_type        : <0x84a>
    <848>   DW_AT_upper_bound : 2
 <2><849>: Abbrev Number: 0
 <1><84a>: Abbrev Number: 4 (DW_TAG_typedef)
    <84b>   DW_AT_type        : <0x86d>
 <1><84f>: Abbrev Number: 0
 <0><85b>: Abbrev Number: 5 (DW_TAG_compile_unit)
    <861>   DW_AT_name        : src/gdb/testsuite/gdb.ada/arraydim/inc.c
 <1><86d>: Abbrev Number: 6 (DW_TAG_base_type)
    <86e>   DW_AT_byte_size   : 8
    <86f>   DW_AT_encoding    : 7       (unsigned)
    <870>   DW_AT_name        : long unsigned int
 <1><874>: Abbrev Number: 7 (DW_TAG_base_type)
    <875>   DW_AT_byte_size   : 4
    <876>   DW_AT_encoding    : 5       (signed)
    <877>   DW_AT_name        : int
 <1><87b>: Abbrev Number: 8 (DW_TAG_variable)
    <87c>   DW_AT_name        : global_3dim_for_gdb_testing
    <882>   DW_AT_type        : <0x832>
    <886>   DW_AT_external    : 1
...

The DWARF contains an anonymous typedef at 0x84a, referring to 0x86d.
Strictly speaking, the anonymous typedef is illegal DWARF, because a
DW_TAG_typedef is defined to have an DW_AT_name attribute containing the name
of the typedef as it appears in the source program.

The DWARF reading code creates a corresponding type for this typedef, which
goes on to confuse the code handling arrays.

Rather than trying to support the type representing this anonymous typedef in
all the locations where it causes problems, fix this by treating the anonymous
typedef as a forwarder DIE in the DWARF reader.

Tested on x86_64-linux, with target boards unix and
unix/-feliminate-dwarf2-dups.

This fixes ~85 failures for unix/-feliminate-dwarf2-dups.

gdb/ChangeLog:

2020-03-07  Tom de Vries  <tdevries@suse.de>

	* dwarf2/read.c (read_typedef): Treat anonymous typedef as forwarder
	DIE.