Make the linespec/location completer ignore data symbols

Make the linespec/location completer ignore data symbols

Currently "b foo[TAB]" offers data symbols as completion candidates.
This doesn't make sense, since you can't set a breakpoint on data
symbols, only on code symbols.

 (gdb) b globa[TAB]
 (gdb) b global [ENTER]
 Function "global" not defined.
 Make breakpoint pending on future shared library load? (y or [n]) n
 (gdb) info symbol global
 global in section .rodata

So this patch makes linespec completion ignore data symbols.

gdb/ChangeLog:
2017-11-08  Pedro Alves  <palves@redhat.com>

	* ada-lang.c (ada_make_symbol_completion_list): Use
	completion_skip_symbol.
	* symtab.c (symbol_is_function_or_method(minimal_symbol*)): New.
	(symbol_is_function_or_method(symbol*)): New.
	(add_symtab_completions): Add complete_symbol_mode parameter.  Use
	completion_skip_symbol.
	(default_collect_symbol_completion_matches_break_on): Use
	completion_skip_symbol.  Pass down mode.
	(collect_file_symbol_completion_matches): Pass down mode.
	* symtab.h (symbol_is_function_or_method): New declarations.
	(completion_skip_symbol): New template function.