Pass existing_cu object to cutu_reader

Pass existing_cu object to cutu_reader

It is possible, seemingly for a special case described in
find_partial_die, for cutu_reader to re-use an existing dwarf2_cu
instead of creating a new one.  This happens when running this test, for
example:

    make check TESTS="gdb.dwarf2/fission-reread.exp"

Right now the, `use_existing_cu` flag tells cutu_reader to use the
dwarf2_cu object at dwarf2_per_cu_data::cu.  However, we'll remove that
field, so we need to find another solution.

This situation arises when some caller up the stack has already created
the dwarf2_cu to read a dwarf2_per_cu_data, but needs to re-read it with
some other parameters.  Therefore, it's possible to just have that
caller pass down the dwarf2_cu object to use as a `existing_cu`
parameter.  If `existing_cu` is NULL, it tells cutu_reader that it needs
to instantiate a new one.

gdb/ChangeLog:

	* dwarf2/read.c (class cutu_reader) <cutu_reader>: Replace
	`int use_existing_cu` parameter with `dwarf2_cu *existing_cu`.
	(init_tu_and_read_dwo_dies): Likewise.
	(cutu_reader::init_tu_and_read_dwo_dies): Likewise.
	(cutu_reader::cutu_reader): Likewise.
	(load_partial_comp_unit): Likewise.
	(process_psymtab_comp_unit): Update.
	(build_type_psymtabs_1): Update.
	(process_skeletonless_type_unit): Update.
	(load_full_comp_unit): Update.
	(find_partial_die): Update.
	(dwarf2_read_addr_index): Update.
	(read_signatured_type): Update.

Change-Id: Id03e3bc3de3cf99d9e4b4080ad83b029c93bf434