[gdb/breakpoint] Handle setting breakpoint on label without address

Consider test-case test.c:
...
$ cat test.c
int main (void) {
  return 0;
 L1:
  (void)0;
}
...

Compiled with debug info:
...
$ gcc test.c -g
...

When attempting to set a breakpoint at L1, which is a label without address:
...
 <1><f4>: Abbrev Number: 2 (DW_TAG_subprogram)
    <f5>   DW_AT_name        : main
 <2><115>: Abbrev Number: 3 (DW_TAG_label)
    <116>   DW_AT_name        : L1
    <119>   DW_AT_decl_file   : 1
    <11a>   DW_AT_decl_line   : 5
 <2><11b>: Abbrev Number: 0
...
we run into an internal-error:
...
$ gdb -batch a.out -ex "b main:L1"
linespec.c:3233: internal-error: void \
  decode_line_full(const event_location*, int, program_space*, symtab*, \
  int, linespec_result*, const char*, const char*): \
  Assertion `result.size () == 1 || canonical->pre_expanded' failed.
A problem internal to GDB has been detected,
further debugging may prove unreliable.
...

Fix this by detecting the error condition in decode_line_full instead, and
throwing an error, such that we have instead:
...
(gdb) b main:L1
Location main:L1 not available
(gdb)
...

Unfortunately, to call event_location_to_string, which is used to get the
location name in the error message, we need to pass a non-const struct
event_location, because the call may cache the string in the struct (See
EL_STRING).  So, we change the prototype of decode_line_full accordingly, and
everywhere this propages to.

Tested on x86_64-linux.

gdb/ChangeLog:

2020-08-28  Tom de Vries  <tdevries@suse.de>

	PR breakpoint/26544
	* breakpoint.c (parse_breakpoint_sals): Remove const from struct
	event_location.
	(create_breakpoint): Same.
	(base_breakpoint_decode_location): Same.
	(bkpt_create_sals_from_location): Same.
	(bkpt_decode_location): Same.
	(bkpt_probe_create_sals_from_location): Same.
	(bkpt_probe_decode_location): Same.
	(tracepoint_create_sals_from_location): Same.
	(tracepoint_decode_location): Same.
	(tracepoint_probe_decode_location): Same.
	(strace_marker_create_sals_from_location): Same.
	(strace_marker_decode_location): Same.
	(create_sals_from_location_default): Same.
	(decode_location_default): Same.
	* breakpoint.h (struct breakpoint_ops): Same.
	(create_breakpoint): Same.
	* linespec.h (decode_line_full): Same.
	* linespec.c (decode_line_full): Same.  Throw error if
	result.size () == 0.

gdb/testsuite/ChangeLog:

2020-08-28  Tom de Vries  <tdevries@suse.de>

	* gdb.base/label-without-address.c: New test.
	* gdb.base/label-without-address.exp: New file.

3 files changed, 66 insertions, 0 deletions

diff --git a/gdb/testsuite/ChangeLog b/gdb/testsuite/ChangeLog
index 6ce2b71..6335ece 100644
--- a/gdb/testsuite/ChangeLog
+++ b/gdb/testsuite/ChangeLog
@@ -1,3 +1,8 @@
+2020-08-28  Tom de Vries  <tdevries@suse.de>
+
+	* gdb.base/label-without-address.c: New test.
+	* gdb.base/label-without-address.exp: New file.
+
 2020-08-27  Pedro Alves  <pedro@palves.net>
 
 	PR gdb/26523
diff --git a/gdb/testsuite/gdb.base/label-without-address.c b/gdb/testsuite/gdb.base/label-without-address.c
new file mode 100644
index 0000000..f0d4a42
--- /dev/null
+++ b/gdb/testsuite/gdb.base/label-without-address.c
@@ -0,0 +1,24 @@
+/* This testcase is part of GDB, the GNU debugger.
+
+   Copyright 2020 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;
+ L1:
+  (void)0;
+}
diff --git a/gdb/testsuite/gdb.base/label-without-address.exp b/gdb/testsuite/gdb.base/label-without-address.exp
new file mode 100644
index 0000000..0fcb1fd
--- /dev/null
+++ b/gdb/testsuite/gdb.base/label-without-address.exp
@@ -0,0 +1,37 @@
+# Copyright 2020 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/>.  */
+
+standard_testfile
+
+if {[prepare_for_testing "failed to prepare" $testfile $srcfile debug]} {
+    return -1
+}
+
+set supported 0
+gdb_test_multiple "l main:L1" "" {
+    -wrap -re "No label \"L1\" defined in function \"main\"\." {
+	unsupported $gdb_test_name
+    }
+    -wrap -re "L1:\r\n.*" {
+	pass $gdb_test_name
+	set supported 1
+    }
+}
+
+if { ! $supported } {
+    return -1
+}
+
+gdb_test "break main:L1" "Location main:L1 not available"