Change increment_reading_symtab to return a scoped_restore

Change increment_reading_symtab to return a scoped_restore

This changes increment_reading_symtab to return a scoped_restore, then
fixes up the users.

gdb/ChangeLog
2017-04-12  Tom Tromey  <tom@tromey.com>

	* symfile.h (increment_reading_symtab): Update type.
	* symfile.c (decrement_reading_symtab): Remove.
	(increment_reading_symtab): Return a scoped_restore_tmpl<int>.
	* psymtab.c (psymtab_to_symtab): Update.
	* dwarf2read.c (dw2_instantiate_symtab): Update.