gdb/completer: improve tab completion to consider the '-force-condition' flag

gdb/completer: improve tab completion to consider the '-force-condition' flag

The commit

  commit 733d554a4625db4ffb89b7a20e1cf27ab071ef4d
  Author: Tankut Baris Aktemur <tankut.baris.aktemur@intel.com>
  Date:   Tue Oct 27 10:56:03 2020 +0100

  gdb/breakpoint: add flags to 'condition' and 'break' commands to force condition

introduced the '-force-condition' flag to the 'break' command.  This
flag was defined as a keyword like 'thread', 'task', and 'if'.
However, it starts with '-'.  This difference caused an uncovered case
when tab-completing a seemingly complete linespec.

Below, we see "-force-condition" in the completion list, where both
the options and the keywords are listed:

  (gdb) break -function main <TAB>
  -force-condition  -function  -label  -line  -qualified
  -source           if         task    thread

But tab-completing '-' lists only options:

  (gdb) break -function main -<TAB>
  -function   -label      -line       -qualified  -source

This patch fixes the problem by adding keywords to the completion
list, so that we see:

  (gdb) break -function main -<TAB>
  -force-condition  -function  -label  -line  -qualified  -source

gdb/ChangeLog:
2020-12-07  Tankut Baris Aktemur  <tankut.baris.aktemur@intel.com>

	* completer.c (complete_explicit_location): Also add keywords
	that start with '-' to the completion list.

gdb/testsuite/ChangeLog:
2020-12-07  Tankut Baris Aktemur  <tankut.baris.aktemur@intel.com>

	* gdb.linespec/explicit.exp: Extend with a test to check completing
	'-' after seemingly complete options.