Move last_source_start_addr to buildsym_compunit

Move last_source_start_addr to buildsym_compunit

This moves the global last_source_start_addr into buildsym_compunit,
adding some accessors as well.

gdb/ChangeLog
2018-07-16  Tom Tromey  <tom@tromey.com>

	* xcoffread.c (aix_process_linenos, complete_symtab): Update.
	* dbxread.c (read_ofile_symtab): Update.
	* coffread.c (coff_start_symtab, coff_end_symtab): Update.
	* buildsym.h (last_source_start_addr): Remove.
	(set_last_source_start_addr, get_last_source_start_addr):
	Declare.
	* buildsym.c (buildsym_compunit::buildsym_compunit): Add last_addr
	parameter.
	(struct buildsym_compunit) <m_last_source_start_addr>: New
	member.
	(prepare_for_building): Remove start_addr parameter.
	(start_symtab, restart_symtab, end_symtab_get_static_block)
	(end_symtab_with_blockvector): Update.
	(set_last_source_start_addr, get_last_source_start_addr): New
	functions.