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

[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.