move the "main" data into the per-BFD object

This adds the "main"-related data into the per-BFD.  This is needed
because once symbol sharing across objfiles is complete, computing the
main name as a side effect of symbol reading will no longer work --
the symbols simply won't be re-read.

After this change, set_main_name is only used by the main_name
machinery itself, so this patch makes it static.

2014-01-15  Tom Tromey  <tromey@redhat.com>

	* dbxread.c (process_one_symbol): Use set_objfile_main_name.
	* dwarf2read.c (read_partial_die): Use set_objfile_main_name.
	* objfiles.c (get_objfile_bfd_data): Initialize language_of_main.
	(set_objfile_main_name): New function.
	* objfiles.h (struct objfile_per_bfd_storage) <name_of_main,
	language_of_main>: New fields.
	(set_objfile_main_name): Declare.
	* symtab.c (find_main_name): Loop over objfiles to find the main
	name and language.
	(set_main_name): Now static.
	(get_main_info): Add comment.
	* symtab.h (set_main_name): Don't declare.