Move last_source file to buildsym_compunit

Move last_source file to buildsym_compunit

This moves the global last_source_file into buildsym_compunit.

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

	* buildsym.c (buildsym_compunit::buildsym_compunit): Add name
	parameter.
	(buildsym_compunit::set_last_source_file): New method.
	<m_last_source_file>: New member.
	(prepare_for_building): Remove "name" parameter.
	(start_symtab, restart_symtab, reset_symtab_globals): Update.
	(last_source_file): Remove.
	(set_last_source_file, get_last_source_file): Update.