Use block_symbol in linespec APIs

This patch changes the linespec.c APIs to use block_symbol instead of just
a symbol.  lookup_symbol et al already return block_symbol's.

gdb/ChangeLog:

	* linespec.c (struct linespec) <function_symbols, label_symbols>:
	Change to vector of block_symbol.  Update all users.
	(struct collect_info) <symbols>: Likewise.
	(collect_info::add_symbol): Take block_symbol as argument.
	Update all callers.
	(decode_compound_collector) <m_symbols>: Change type to vector
	of block_symbol.  Update all users.
	(decode_compound_collector::operator ()): Change parameter type
	to block_symbol.
	(find_method, find_function_symbols, find_linespec_symbols)
	(find_label_symbols_in_block, find_label_symbols): Change symbol
	vectors to block_symbol vectors.
	* symtab.h (symbol_found_callback_ftype): Change parameter type to
	block_symbol.

gdb/ada-lang.c

@@ -5836,7 +5836,7 @@ ada_iterate_over_symbols
 
   for (i = 0; i < ndefs; ++i)
     {
-      if (!callback (results[i].symbol))
+      if (!callback (&results[i]))
 	break;
     }
 }