Make queue_and_load_dwo_tu receive a dwarf2_cu

queue_and_load_dwo_tu, used as a callback for htab_traverse_noresize,
currently receives a dwarf2_per_cu_data as its `info` user data.  It
accesses the current dwarf2_cu object through the dwarf2_per_cu_data::cu field.
This field will be removed, because the dwarf2_per_cu_data will become
objfile-independent, while dwarf_cu will remain objfile-dependent.

To remove references to this field, change queue_and_load_dwo_tu so
that it expects to receive a pointer to the dwarf2_cu as its info
parameter.

A reference to dwarf2_per_cu_data::cu needs to be added, but it will get
removed in a subsequent patch, when this function gets re-worked.

I kept this as a separate patch, because since there's no strong typing
here, it's easy to miss something.

gdb/ChangeLog:

	* dwarf2/read.c (queue_and_load_dwo_tu): Expect a dwarf2_cu as
	the info parameter.
	(queue_and_load_all_dwo_tus): Pass per_cu->cu.

Change-Id: I3db2a780f0e2157d52ce6939f478558ffe20abcf