Split up end_symtab_from_static_block into two.

Split up end_symtab_from_static_block into two.

This patch is conceptually quite simple.
If you look at end_symtab_from_static_block you'll see
that the static_block == NULL case is completely different
than the non-NULL case.

There's a lot of complexity to handle the NULL case but it seems
entirely unnecessary.  For example, whether blockvector is NULL
is decided at the start, before this for loop:

  for (subfile = subfiles; subfile; subfile = nextsub)

Secondly, after the for loop, we test symtab for non-NULL here:

  /* Set this for the main source file.  */
  if (symtab)

but symtab will only ever be non-NULL if blockvector was non-NULL.
And if blockvector was non_NULL so will symtab.

The other case to consider is these lines of code executed before
the for loop:

  /* Read the line table if it has to be read separately.
     This is only used by xcoffread.c.  */
  if (objfile->sf->sym_read_linetable != NULL)
    objfile->sf->sym_read_linetable (objfile);

  /* Handle the case where the debug info specifies a different path
     for the main source file.  It can cause us to lose track of its
     line number information.  */
  watch_main_source_file_lossage ();

From my reading of the code, neither of these is useful
in the static_block == NULL case.

Thus we can make the code more readable by splitting these two cases up,
which is what this patch does.

gdb/ChangeLog:

	* buildsym.c (main_subfile): New static global.
	(free_subfiles_list): New function.
	(start_symtab): Set main_subfile.
	(restart_symtab): Replace init of subfiles, current_subfile with
	call to free_subfiles_list.
	(watch_main_source_file_lossage): Use main_subfile.
	(reset_symtab_globals): Replace init of current_subfile with call
	to free_subfiles_list.
	(end_symtab_without_blockvector, end_symtab_with_blockvector): New
	functions, split out from ...
	(end_symtab_from_static_block): ... here.  Rewrite to call them.