gdb: Add a class to track last display symtab and line information

gdb: Add a class to track last display symtab and line information

In stack.c we currently have a set of static global variables to track
the last displayed symtab and line.  This commit moves all of these
into a class and adds an instance of the class to track the same
information.

The API into stack.c is unchanged after this cleanup.

There should be no user visible changes after this commit.

gdb/ChangeLog:

	* stack.c (set_last_displayed_sal): Delete.
	(last_displayed_sal_valid): Delete.
	(last_displayed_pspace): Delete.
	(last_displayed_addr): Delete.
	(last_displayed_symtab): Delete.
	(last_displayed_line): Delete.
	(class last_displayed_symtab_info_type): New.
	(last_displayed_symtab_info): New static global variable.
	(print_frame_info): Call methods on last_displayed_symtab_info.
	(clear_last_displayed_sal): Update header comment, and make use of
	last_displayed_symtab_info.
	(last_displayed_sal_is_valid): Likewise.
	(get_last_displayed_pspace): Likewise.
	(get_last_displayed_addr): Likewise.
	(get_last_displayed_symtab): Likewise.
	(get_last_displayed_line): Likewise.
	(get_last_displayed_sal): Likewise.
	* stack.h (clear_last_displayed_sal): Update header comment.
	(last_displayed_sal_is_valid): Likewise.
	(get_last_displayed_pspace): Likewise.
	(get_last_displayed_addr): Likewise.
	(get_last_displayed_symtab): Likewise.
	(get_last_displayed_line): Likewise.
	(get_last_displayed_sal): Likewise.

Change-Id: Ia3dbfe267feec03108c5c8ed8bd94fc0a030c3ed