Add dwarf2_per_cu_data::index

Add dwarf2_per_cu_data::index

Currently, a dwarf2_per_cu_data can hold a link to the corresponding
expanded compunit_symtab.  However, the dwarf2_per_cu_data objects are
shared across objfiles, a simple pointer won't work: each objfile
sharing the dwarf2_per_cu_data instance will have a corresponding
compunit_symtab.

Instead, this link will be stored in the dwarf2_per_objfile object
(which will contain the objfile-specific data).  To enable this, we add
an index to each dwarf2_per_cu_data and signatured_type.  The data
structure in the dwarf2_per_objfile will use this new index to map a
dwarf2_per_cu_data to its corresponding compunit_symtab, for this
objfile.

gdb/ChangeLog:

	* dwarf2/read.h (struct dwarf2_per_objfile) <allocate_per_cu,
	allocate_signatured_type>: Declare new methods.
	<m_num_psymtabs>: New member.
	(struct dwarf2_per_cu_data) <index>: New member.
	* dwarf2/read.c (dwarf2_per_objfile::allocate_per_cu)
	(dwarf2_per_objfile::allocate_signatured_type): New methods.
	(create_cu_from_index_list): Use allocate_per_cu.
	(create_signatured_type_table_from_index)
	(create_signatured_type_table_from_debug_names)
	(create_debug_type_hash_table, add_type_unit)
	(read_comp_units_from_section): Use allocate_signatured_type.

Change-Id: I7733479a38ce82a5015cb184c8acce5f8bbf2e69