gdb: fix until behavior with trailing !is_stmt lines

gdb: fix until behavior with trailing !is_stmt lines

When using the command "until", it is expected that GDB will exit a
loop if the current instruction is the last one related to that loop.
However, if there were trailing non-statement instructions, "until"
would just behave as "next".  This was noticeable in clang-compiled
code, but might happen with gcc-compiled as well.  PR gdb/17315 relates
to this problem, as running gdb.base/watchpoint.exp with clang
would fail for this reason.

To better understand this issue, consider the following source code,
with line numbers marked on the left:

  10:	for (i = 0; i < 10; ++i)
  11:     loop_body ();
  12:   other_stuff ();

If we transform this to pseudo-assembler, and generate a line table,
we could end up with something like this:

  Address | Pseudo-Assembler | Line | Is-Statement?

  0x100   | i = 0            | 10   | Yes
  0x104   | loop_body ()     | 11   | Yes
  0x108   | i = i + 1        | 10   | Yes
  0x10c   | if (i < 10):     | 10   | No
  0x110   |     goto 0x104   | 10   | No
  0x114   | other_stuff ()   | 12   | Yes

Notice the two non-statement instructions at the end of the loop.

The problem is that when we reach address 0x108 and use 'until',
hoping to leave the loop, GDB sets up a stepping range that runs from
the start of the function (0x100 in our example) to the end of the
current line table entry, that is 0x10c in our example.  GDB then
starts stepping forward.

When 0x10c is reached GDB spots that we have left the stepping range,
that the new location is not a statement, and that the new location is
associated with the same source line number as the previous stepping
range.  GDB then sets up a new stepping range that runs from 0x10c to
0x114, and continues stepping forward.

Within that stepping range the inferior hits the goto (at 0x110) and
loops back to address 0x104.

At 0x104 GDB spots that we have left the previous stepping range, that
the new address is marked as a statement, and that the new address is
for a different source line.  As a result, GDB stops and returns
control to the user.  This is not what the user was expecting, they
expected GDB to exit the loop.

The fix proposed in this patch, is that, when the user issues the
'until' command, and GDB sets up the initial stepping range, GDB will
check subsequent SALs (symtab_and_lines) to see if they are
non-statements associated with the same line number.  If they are then
the end of the initial stepping range is extended to the end of the
non-statement SALs.

In our example above, the user is at 0x108 and uses 'until', GDB now
sets up a stepping range from the start of the function 0x100 to
0x114, the first address associated with a different line.

Now as GDB steps around the loop it never leaves the initial stepping
range.  It is only when GDB exits the loop that we leave the stepping
range, and the stepping finishes at address 0x114.

This patch also adds a test case that can be run with gcc to test that
this functionality is not broken in the future.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=17315