Change `function_symbols' to std::vector

Change `function_symbols' to std::vector

This patch changes the `function_symbols'  members in linespec.c structures
from a VEC to a std::vector.

gdb/ChangeLog:

	* linespec.c (struct linespec) <function_symbols>: Change type to
	std::vector.  Update all users.
	(struct collect_info) <function_symbols>: Likewise.
	(convert_linespec_to_sals): Use std::sort to sort function_symbols.
	(std_compare_symbols): New function.
	(find_method, find_function_symbols, find_linespec_symbols)
	(find_label_symbols_in_block): Change `symbols' parameter to
	std::vector.  Update all callers.
	(find_label_symbols): Likewise for `function_symbols' and
	`label_funcs_ret'.