Move pending_blocks and pending_block_obstack to buildsym_compunit

Move pending_blocks and pending_block_obstack to buildsym_compunit

This moves the pending_blocks and pending_block_obstack into
buildsym_compunit.

The obstack could perhaps be merged with the addrmap obstack, but I
did not do that in this series.

gdb/ChangeLog
2018-07-20  Tom Tromey  <tom@tromey.com>

	* buildsym.h (class scoped_free_pendings): Remove constructor.
	* buildsym.c (struct buildsym_compunit) <free_pending_blocks>: New
	method.
	<m_pending_block_obstack, m_pending_blocks>: New members.
	(pending_block_obstack, pending_blocks): Remove.
	(scoped_free_pendings::scoped_free_pendings): Default.
	(~scoped_free_pendings): Update.
	(free_pending_blocks): Remove.
	(finish_block_internal, record_pending_block, make_blockvector)
	(end_symtab_get_static_block, augment_type_symtab, push_context)
	(buildsym_init): Update.