Constify main_name

This patch constifies the return type of main_name.  There is a
comment indicating that this wasn't possible at some point in the
past, but whatever the barrier was, it is gone now.

Tested by rebuilding.

gdb/ChangeLog
2019-07-18  Tom Tromey  <tromey@adacore.com>

	* symtab.c (main_name): Constify return type.
	* symfile.c (set_initial_language): Update.
	* symtab.h (main_name): Constify return type.

gdb/symtab.c

@@ -5741,8 +5741,10 @@ find_main_name (void)
   set_main_name ("main", language_unknown);
 }
 
-char *
-main_name (void)
+/* See symtab.h.  */
+
+const char *
+main_name ()
 {
   struct main_info *info = get_main_info ();