Move current_subfile to buildsym_compunit

Move current_subfile to buildsym_compunit

This moves the global current_subfile into buildsym_compunit.

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

	* xcoffread.c (process_linenos): Update.
	* stabsread.c (define_symbol, read_type, read_enum_type): Update.
	* mdebugread.c (psymtab_to_symtab_1): Update.
	* dwarf2read.c (setup_type_unit_groups)
	(lnp_state_machine::handle_set_file, dwarf_record_line_p)
	(lnp_state_machine::record_line, dwarf_decode_lines): Update.
	* dbxread.c (process_one_symbol): Update.
	* coffread.c (coff_symtab_read, enter_linenos)
	(process_coff_symbol): Update.
	* buildsym.h (current_subfile): Don't declare.
	(get_current_subfile): Declare.
	* buildsym.c (struct buildsym_compunit) <m_current_subfile>: New
	member.
	(start_subfile, free_buildsym_compunit, push_subfile)
	(prepare_for_building, start_symtab): Update.
	(get_current_subfile): New function.