Constify ada_main_name

Unlike the other *_main_name functions, ada_main_name returns a
non-const "char *".  This is strange, though, because the caller
should not in fact modify or free this pointer.  This patch changes
this function to constify its return type.

gdb/ada-lang.c

@@ -805,8 +805,8 @@ ada_update_initial_language (enum language lang)
    The result is good until the next call.  Return NULL if the main
    procedure doesn't appear to be in Ada.  */
 
-char *
-ada_main_name (void)
+const char *
+ada_main_name ()
 {
   struct bound_minimal_symbol msym;
   static gdb::unique_xmalloc_ptr<char> main_program_name;

gdb/ada-lang.h

@@ -337,7 +337,7 @@ extern enum ada_renaming_category ada_parse_renaming (struct symbol *,
 
 extern void ada_find_printable_frame (frame_info_ptr fi);
 
-extern char *ada_main_name (void);
+extern const char *ada_main_name ();
 
 extern void create_ada_exception_catchpoint
   (struct gdbarch *gdbarch, enum ada_exception_catchpoint_kind ex_kind,