Fix symtab/23853: symlinked default symtab

Fix symtab/23853: symlinked default symtab

This patch attempts to fix a bug dealing with setting breakpoints
in default symtabs that are symlinks.  For example:

(gdb) list
11	   GNU General Public License for more details.
12
13	   You should have received a copy of the GNU General Public License
14	   along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
15
16	static int
17	foo (void)
18	{
19	  return 0; /* break here  */
20	}
(gdb)
21
22	int
23	main (void)
24	{
25	  return foo ();
26	}
(gdb) b 19
No line 19 in the current file.
Make breakpoint pending on future shared library load? (y or [n])

The problem here is that when create_sals_line_offset sets the default
symtab, it immediately calls symtab_to_fullname, passing that fullname
to collect_symtabs_from_filename to find all matching symtabs.  This
fails because we end up looking for a symtab with the name of the
actual file on disk (which is different in this case because of the
symlink) instead of the one stored in the debug info.

Since we already have the lookup name of the default symtab, use it
instead of the fullname. [This fullname thing was originally added
in 2007 in a series dealing with *displaying* absolute file names.
Clearly, this instance has nothing to do with the display of file names.]

gdb/ChangeLog

	PR symtab/23853
	* linespec.c (create_sals_line_offset): Search for the default
	symtab's filename instead of its fullname.

gdb/testsuite/ChangeLog

	PR symtab/23853
	* gdb.base/symlink-sourcefile.c: New file.
	* gdb.base/symlink-sourcefile.exp: New file.