Change decode_compound_collector to use std::vector

Change decode_compound_collector to use std::vector

This patch changes decode_compound_collector to use std::vector instead of
VEC, eliminating a cleanup in the process.

gdb/ChangeLog:

	* linespec.c (decode_compound_collector::decode_compound_collector):
	Remove initialization for `m_symtabs'.
	(decode_compound_collector::release_symbols): Change return type
	to std::vector.  Update all callers.
	(class decode_compound_collector) <m_symbols>: Change type to
	std::vector.
	(lookup_prefix_sym): Change return type to std::vector.  Update all
	callers.
	(compare_symbols): Remove.
	(std_compare_symbols): Rename to `compare_symbols'.
	(find_method): Change `sym_classes' parameter to std::vector.
	Update all callers.  Use std::sort to sort sym_classes.
	(find_linespec_symbols): Remove cleanup.