Remove DWARF queue-related globals

Remove DWARF queue-related globals

This removes some queue-related globals from the DWARF reader, in
favor of a new member on dwarf2_per_objfile.  Globals must be avoided
in this code, because they prevent multi-threading the reader.

gdb/ChangeLog
2020-02-08  Tom Tromey  <tom@tromey.com>

	* dwarf2/read.h (struct dwarf2_queue_item): Move from
	dwarf2/read.c.  Remove "next" member.  Add constructor ntad
	destructor.
	(struct dwarf2_per_objfile) <queue>: New member.
	* dwarf2/read.c (struct dwarf2_queue_item): Move to
	dwarf2/read.h.
	(dwarf2_queue, dwarf2_queue_tail): Remove.
	(class dwarf2_queue_guard): Add parameter to constructor.  Use
	DISABLE_COPY_AND_ASSIGN.
	<m_per_objfile>: New member.
	<~dwarf2_queue_guard>: Rewrite.
	(dw2_do_instantiate_symtab, queue_comp_unit, process_queue):
	Update.
	(~dwarf2_queue_item): New.

Change-Id: Ied1f6ff3691352a66c4709b0b2cba0588f49f79a