[gdb/symtab] Add optimized out static var to cooked index

Consider the test-case:
...
$ cat main.c
int main (void) { return 0; }
$ cat static-optimized-out.c
static int aaa;
...
compiled like this:
...
$ gcc-12 static-optimized-out.c main.c -g -O2 -flto
...

There's a difference in behaviour depending on symtab expansion state:
...
$ gdb -q -batch a.out -ex "print aaa"
No symbol "aaa" in current context.
$ gdb -q -batch a.out -ex "maint expand-symtab" -ex "print aaa"
$1 = <optimized out>
...

The reason for the difference is that the optimized out variable aaa:
...
 <1><104>: Abbrev Number: 2 (DW_TAG_variable)
    <105>   DW_AT_name        : aaa
    <109>   DW_AT_decl_file   : 1
    <10a>   DW_AT_decl_line   : 18
    <10b>   DW_AT_decl_column : 12
    <10c>   DW_AT_type        : <0x110>
...
is not added to the cooked index because of this clause in abbrev_table::read:
...
     else if (!has_location && !has_specification_or_origin && !has_external
	       && cur_abbrev->tag == DW_TAG_variable)
	cur_abbrev->interesting = false;
...

Fix this inconsistency by making sure that the optimized out variable is added
to the cooked index.

Regression tested on x86_64-linux.

Add two test-cases, a C test-case gdb.opt/static-optimized-out.exp and a dwarf
assembly test-case gdb.dwarf2/static-optimized-out.exp.

Tested gdb.opt/static-optimized-out.exp with gcc-8 to gcc-12, for which we now
consistently get:
...
(gdb) print aaa^M
$1 = <optimized out>^M
...
and with gcc 7.5.0 and clang 13.0.1, for which we still consistently get:
...
(gdb) print aaa^M
No symbol "aaa" in current context.^M
...
due to missing debug info for the variable.

PR symtab/30656
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=30656

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

1 files changed, 22 insertions, 0 deletions

diff --git a/gdb/testsuite/gdb.opt/main.c b/gdb/testsuite/gdb.opt/main.c
new file mode 100644
index 0000000..c6beff7
--- /dev/null
+++ b/gdb/testsuite/gdb.opt/main.c
@@ -0,0 +1,22 @@
+/* This testcase is part of GDB, the GNU debugger.
+
+   Copyright 2023 Free Software Foundation, Inc.
+
+   This program is free software; you can redistribute it and/or modify
+   it under the terms of the GNU General Public License as published by
+   the Free Software Foundation; either version 3 of the License, or
+   (at your option) any later version.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
+
+int
+main (void)
+{
+  return 0;
+}