Explicit locations -label completer

Explicit locations -label completer

We're missing a completer for

  (gdb) break -function func -label [TAB]

This patch adds one.  Tests will be added later in the series.

gdb/ChangeLog:
2017-07-17  Pedro Alves  <palves@redhat.com>

	* completer.c (collect_explicit_location_matches): Handle
	MATCH_LABEL.
	(convert_explicit_location_to_linespec): New, factored out from
	...
	(convert_explicit_location_to_sals): ... this.
	(complete_label): New.
	(linespec_complete_label, find_label_symbols_in_block): New.
	(find_label_symbols): Add completion_mode parameter and adjust to
	call find_label_symbols_in_block.
	* linespec.h (linespec_complete_label): Declare.