gdb: Add constructor to struct cu_partial_die_info

gdb: Add constructor to struct cu_partial_die_info

Adds a constructor to 'struct cu_partial_die_info' and disables the
default constructor, preventing partially initialised instances from
being created.

Update 'find_partial_die' to return a const struct.

Users of 'find_partial_die' are updated to take account of the above
two changes.

There should be no user visible changes after this commit.

gdb/ChangeLog:

	* dwarf2read.c (struct cu_partial_die_info): Add constructor,
	delete default constructor.
	(find_partial_die): Update to return const struct.
	(partial_die_parent_scope): Move variable declaration into scope
	of its use and change its type to auto.
	(guess_partial_die_structure_name): Likewise.
	(partial_die_info::fixup): Likewise.